Stumped on a furnace issue

Factual Questions
1

My furnace rattles loudly, but only at night, and you can only hear it upstairs. It sounds like a spoon you accidentally left in a half-speed blender, except it’s a steady beat, not chaotic. It starts when the furnace starts at night, and does not go away until I turn the furnace off. The furnace works and blows hot air while the noise is happening. If you ever watch television, it’s at least as loud as that. It’s impossible to sleep through. Here’s everything I’ve done to diagnose the issue so far:

  1. If I place my ear to the wall at any point upstairs, it sounds as if it’s coming from that exact point, on the other side of the wall. This is true no matter where I test it upstairs, but not downstairs. No matter where I put my ear, it sounds like the source of the rattling.

  2. It’s not coming from the furnace itself. When I climbed up into the attic to try to find the noise, there was no rattling noise even though it was very loud and clear as soon as I came back down the ladder.

  3. This makes me suspect it’s not the furnace making the noise, but something in the duct work.This would totally make sense except that…

  4. It only happens at night. When the furnace blows hot air during the day, there is no rattling. The noise almost seems malicious, like it’s specifically designed to only happen while we’re trying to sleep.

Anyone have any ideas? I’m going crazy. The next step is to just burn the place down and collect the insurance money so we can move somewhere that never gets cold.

2

For the moment, let’s assume the noise is not coming from the furnace.

Based on what you said, I’d agree that it’s coming from the ductwork, assuming you have sheet metal ducts. If it’s only happening at night, I’m going to guess that something is contracting at night (when it’s cold) and leaving a flap of sheet metal exposed, just right, so the moving air makes it flap.

What bugs me about that, however, is that if it’s happening at night due to the sheet metal contracting, I’d think it would stop after the furnace warms them back up. OTOH, it could be interacting with some of the structure that it’s mounted to (ie rafters/joists/studs/drywall etc).

What I think you should do next is isolate it. First, just for kicks, take the panel off your furnace and make sure the squirrel cage spins freely and makes no noise. Turn it on and check that way as well (you’ll likely need to hold down a safety switch to get it going.
If you’re right about what you said earlier, this will totally eliminate the furnace. It’ll be even better if you can hear the noise while you’re doing this and be 100% sure it’s not making the noise.

I’d also try running the fan only and see if that changes anything. If not, that would rule out anything to do with heat or any of the heating components (inducer blower, heat exchange…).

You probably (well, maybe) have a fresh air intake in the system somewhere. An insulated flex duct that goes to the outside. Make sure that it’s not that, especially since it interacts directly with the outdoor air.

Next, I’d start closing/opening branches of the heating system. Run the furnace (or just the blower if that works) and while you can hear the noise, find the dampers, note the angle that they’re at, and try fully closing or opening them (and return them to where they started when you’re done). You might even just be able to open and close individual vents in different rooms. With any luck, close or opening one of those dampers or vents will make the noise stop. If that happens, you can go from there.

One other thing, make sure not of your vents are doing it. On some of mine, if they’re open at juuust right amount, they’ll rattle and make a metal on metal knocking sound.

3

Are the upstairs and downstairs heated through the same duct or are they separate branches off of the heater?

Do you have an inline duct fan?

4

Thanks! I’ll try that. For what it’s worth, I did open the panel and the furnace kept running while I was looking inside. Couldn’t hear the noise at the time. Not sure if some previous tech bypassed a safety switch or something, but I never had to hold one down.

I have tried this. The noise doesn’t happen while the fan is running without the furnace on.

How can I find this? If something like this existed, wouldn’t it need to be connected to a filter? The only filter I ever change comes from an intake on the inside.

I don’t know how to access any dampers or what they really look like. I see pictures of what they’re supposed to look like on google, but I can’t find anything like that in the areas I can access.

I just finished watching this video https://www.youtube.com/watch?v=LTH_RMPgp3k, and by the sound the damper makes when he turns it, this sure seems like a prime suspect. Especially since it only rattles upstairs. I can imagine that a rattle at the damper in the duct that feeds the upstairs would isolate the noise to just that area. I’m just stuck on how to actually find it!

The duct work is mostly inaccessible (at least at my level of inexperience) due to space restrictions in my attic, except for the area immediately surrounding the furnace. That’s the only area that has an actual flat surface for me to step on.

Also, if this was the issue, wouldn’t I be able to hear it while poking around in the attic? The noise was clearly happening upstairs until I went to physically inspect the furnace. Pretty much as soon as my body was halfway up the ladder, I couldn’t hear it anymore. I even went up and down a few steps, to make sure the noise was actually still happening and didn’t just happen to stop while I was on the ladder. It didn’t.

Maybe the damper rattles around inside the vent, but the duct acts like a kind of megaphone? Except then, why can I hear it much more clearly by putting my ear against the wall?

I did check all the vents in my rooms prior to posting this. It’s definitely not the source of the issue.

I really appreciate your insight here. You have given me a bit of hope and I’ll let you know if I finally isolate the issue.

5

I don’t know how to tell if I have an inline duct fan. It seems like I must though, because what other way could it work? Sorry, I’m pretty close to 100% ignorant on this.

There are two ducts, one for upstairs and one for downstairs.

6

An inline duct fan would be a secondary fan that is in the ductwork away from the heater.

There’s no particular reason to think that you’d have one but if you did have one in the upstairs branch it might be what’s making the noise.

When you say that it only.happens at night, what exactly do you mean? Does it begin immediately after sunset, after a certain time on the clock, or what? When does it stop? Is it doing it when you first wake up in the morning?

Are you certain that it’s only at night? Could it be that the only time you’re upstairs is at night, so you don’t hear it during the day?

7

Furnaces typically have two panels. A top one (but if your’s is laying down in the attic it would be closer to the end the air comes out of) that can be removed anytime and a lower panel, which comes off after the first one. The lower one houses the motor, that’s where the switch is. If you took that one off, you’d be looking right at the main blower.

Curiouser and curiouser, only at night and only with the heat going. I assume it doesn’t happen with the AC or you have have mentioned that it was happening in the summer. Of course it’s not cold outside during summer and that may play into it.

It is actually filtered. If you have one, it runs from the outside of your house to a return air duct.
It’s not really part of the furnace, per say, it’s actually make up air for your house. Any time you turn on an exhaust fan (kitchen/bathroom), air has to be brought back in. As houses are built tighter, there’s not many drafty areas to bring in that air. What has been done for the last 20 or so years is to run a vent from the outside of the house to a return duct. Now, as you exhaust air, you’re also bringing in air from the outside and running it through the furnace filter.
You may very well not have one, but if you did, you’ll probably find a small (6x6 inch or so) vent on the outside of your house somewhere, with a small duct coming from it.
It’s probably not where the noise is coming from, just something else to look at.

The dampers could be anywhere in the system or even non-existent, though they’re usually kinda near the furnace, where the ducts branch off the plenum.
They may not have a big red handle on them. Sometimes it’s just a little wingnut that doesn’t seem like it should be there.

One would think that if the damper was making noise, you’d be able to hear it right there. Even if it traveled, the ducts would have to be really well insulated to not hear it at the source.

I’m going to guess it’s in the walls somewhere. Probably just the ducts expanding, or trying to expand, especially if they’re in a tight spot that may have contracted from the weather (like an exterior wall).

At this point, you’re probably going to have to keep trying to locate it the hard way. Since you can’t find any dampers, it might be easier to just start closing individual vents or combinations of vents. If you can get it to stop, you’ll be a lot closer than you are now.

8

To guarantee no airflow, cover individual outlets with plastic - when they did duct cleaning at our house, they took a roll of plastic like sandwich wrap, pulled out each duct end and wrapped it on plastic and put it back. Definitely ensures no airflow on that part.

9

What type of furnace is it? i.e gas or electric

If it’s gas, is there an exhaust duct?

Do you have a smart thermostat that would modulate fan speed?

10

I had a very similar problem. It did not occur only at night, but that’s when I noticed it. The noise was in my bedroom and I don’t just hang out there during the day. Also it happened only when the furnace started up, not the whole time it was blowing.

It seems that the furnace vibrates when it starts up; this seems to be related somehow to the pre-ignition blower cycle. Then it causes some ductwork to vibrate. The ductwork was vibrating a piece of sheet metal in the attic. I put some insulation between the duct and sheet metal to dampen the vibration and that did the trick.

11

On the only at night issue, are there other things you do only at night such as shut a few doors or other things that should/would alter air flow through the house?

12

This really sounds like it is a fan. I don’t know what type of fan you have, a squirrel cage, axial fan, centrifugal, etc., but somewhere there is a fan, and the blades are hitting something. Might happen when the fan blades cool, might be when they warm up, but it is probably a fan.

I had this issue with a pellet stove, not a furnace but the principal is the same, to move the air through the system. In my case the fan was getting old and maybe the blades were fatigued by the heating and cooling, but they got just a little out of line and were hitting the shroud. Ting, ting, ting, ting.

13

That fan that runs from the time the t-stat calls for heat (before the burners or main blower starts) until the heating cycle is over is the inducer fan. It’s the one that brings in fresh (outdoor or from the same room) air, pushes it through the heat exchange and out the exhaust. It’s also the only blower (and it’s blades) to take on the heat of the furnace. The main blower is before the heat exchange.

Also notable, the inducer motor doesn’t run when you just have the t-stat set to fan-on and it doesn’t run with the AC. It’s only for heat.

Having said all that, if it’s the inducer motor, I’d expect the sound to be very noticeable right there by the motor itself. On the other hand, perhaps the noise is coming from the exhaust. The OP could check the exhaust from the furnace to wherever it leaves to see if there’s any noise.

One other thing…and this is important…next time your heat turns on, see if the noise starts before or after air is blowing from the vents.
When you call for heat, a few things happen, in a certain order:

1)Inducer motor turns on. It makes a small whirring sound, but doesn’t move any air in the ducts. It runs for a few seconds.
2)Burners fire, they run for 20-30 seconds or so.
3)Main blower turns on and air moves through the ducts.

If the noise starts before step 3, it’ll rule out the main blower and all of the ductwork.

14

My guess is that the furnace has a thermostatically controlled damper that closes due to a bi-metallic coil when the outside temperature is below a certain point. The pressure from the induction blower is sufficient to open the damper, but closes when the blower is off to prevent warm air from being sucked into the cold night. I suspect that a spring or linkage on that damper is broken, allowing the damper to swing in the flow of the exhaust, causing a “ting ting ting…” all night long while the furnace is running. When the outside air temp is high enough not to need the damper to close, the bi-metallic coil keeps the damper open regardless of whether or not the induction blower is running, so it doesn’t make a noise.