Occasionally when my gas furnace turns on, I get this loud grinding noise coming from my second-floor bedroom wall, an interior wall. It is very inconsistent. Sometimes it has a sound like a kid with a playing card in his bike spokes. It’s sometimes even and sometimes irregular. Some days I don’t notice it at all but sometimes it wakes me up at night. Sometimes it lasts for 30 seconds, sometimes 5 minutes.
The first time I noticed it, it seemed to happen only just before the furnace turned on, which I think is a pre-ignition cycle where the furnace blows out standing air to eliminate any danger of accumulated gas. No air comes through the vents in this cycle, but you can hear a motor running; maybe it just vents air out the flue. The noise would start, then it would stop when the blower would start up and air started coming through the vents.
Now it makes noise at all random times.
The noise can only be heard in the wall on the second floor. When I go to the room immediately below, I don’t hear the noise. The furnace is in the basement below that. This suggests it originates near the bedroom wall, rather than down in the furnace and propagating up the ducts.
I climbed into the attic today. There are two lines running up along the wall where the noise occurs. One is a round metal duct. This may be the flue, not sure. The other is a PVC (or other plastic) pipe with a motor spliced inline. The motor is labelled as a FanTech FR100, and seems to be some sort of exhaust fan. I do not know if this is related to the HVAC or is connected to the bathroom exhaust fan on the floor below. There is a wall switch that turns it on and off. It was running when I was up there but I don’t know if there is any other switch upstream that may also control it.
The fan is the only mechanical device I could see that could be responsible for this, although due to the irregularity of the noise it’s hard to catch it in the act. The fan is 22 years old (I bought the house new in September 1995), and the HVAC system was replaced about three years ago.
Dunno. But if the HVAC fan goes into reverse first to exhaust gasses, it could be that it’s sucking a loose something into the blades, making the noise. When it reverses back again, the whatever it is would be thrown back out of the blades again.
The pre-ignition fan is the inducer fan. It is a small (4" or so) fan that pulls air through the furnace plenum to remove any lingering fumes. It runs on a timer, usually about 45 seconds. If it is exhausting properly, indicated by tripping a vacuum switch, it allows the furnace main gas valve to open and ignition to occur. When the plenum heats up enough to trip the lower blower limit, the main blower comes on.
The inducer fan just makes a faint whirring noise. I suppose it could have a piece of gasket or something sticking out to make a noise, but the noise would be in the furnace.
My guess is the inducer as well, or something to do with it. If you ONLY hear it one specific spot in your house and don’t hear it near the furnace itself (when it fires up), I’d guess there’s something caught in your exhaust pipe. As for why it only happens at the beginning of the cycle and only randomly, is anyone’s guess and hard to say without being on site. Could be anything from something caught on a sheet metal screw that has to be just right to flap around, to something temperature related that changes when the pipe heats up or even just something that vibrates against something else when the conditions are just right. Who knows.
Now, the PVC pipe with the inline motor, that’s something different. Assuming you don’t have a high efficiency/condensing furnace (noted by one or two PVC pipes entering/exiting typically (by code?) exhausting from a side wall, not the roof). I’m going to guess/assume the motor is always running, right? That’s probably a radon exhaust fan. If you poke around in your basement, you’ll probably find the PVC pipe dives into the concrete floor, right? If the motor looks like this, that’s what it is and while I won’t rule out, from here, that it has nothing to do with the noise, it’s not part of the heating system, or at least it shouldn’t be.
I don’t have much expierence with radon fans, but I’m guessing the PVC pipe in the basement will look very much like a plumbing stack.
NETA, extremely minor nitpick, the inducer doesn’t push air through the plenum, it pushes air through the exhaust pipe. It serves two purposes. It, as you said, clears out any lingering gas and it also makes sure the exhaust (and intake for the matter) isn’t blocked. The plenum is the heated/conditioned air that gets blown into the living space.
If you have have a situation where the inducer turns on, then then the furnace shuts down before the gas valve opens up, one of the first things to check is that nothing is blocking the intake or exhaust. Other than a bad vacuum/pressure sensor (or board, wiring etc) a common problem is snow packed against the pipes.
That photo is *exactly *what I see in the attic. Down in the basement near the sump pump I can hear a sound that I thought was the radon fan but maybe it is just the sound of air being sucked in by the fan in the attic. This makes the noise even more perplexing. Maybe this fan is just a red herring, but it’s the only mechanical thing up at that level.
The sound is not a screw rattling around; that would be too random. I’ll see if I can get a decent recording, if I don’t solve it first.
Forced air.
I am trying to force the sound to occur so I can get up in the attic and see if I can trace it. But it is intermittent and not reproducible. It will wake me up at 2:30 but I can’t make it happen when I am fully dressed with a ladder set up to the attic. I have tried all kinds of sequences on the thermostat: Start with the system off (the furnace continues to blow air for quite a while even after the thermostat is set to “off”), with the system on but during the “off” part of the cycle, with the system running. Set the heat 1 degree above indicated temp, 10 degrees above indicated temp. I cannot find a sequence guaranteed to do this.
CookingWithGas - since the sound coincided (At least initially) with the furnace turning on - I suspect it has to do with thermal expansion/contraction effects. See Oil Canning.
Also you mention that it happens more at night when the temperatures are lower, so it lets that more credence.
I doubt it is something stuck in there since that seems to be a straight run and things get stuck at bends.
Stiffening the line with some angle iron could be a solution. Filling the wall withslow rise insulation could be another solution.
I have still not been able to get into the attic when the noise is occurring. Sometimes it does not last long enough. It’s like a video game. When it does last long enough, it’s when I’m in bed and I think, “It’s not worth getting up for, it’ll stop by the time I get up and climb up there” but then it just keeps going.
I think the fan is a red herring. I suspect that something is causing the metal exhaust duct to vibrate, and somehow the vibration is triggering the noise. At one one point the duct passes through a piece of sheet metal about 18" x 18". That piece of metal rests on some of the framing. I have no idea what the metal is for, except possibly to prevent the duct from contacting the framing, which might be a fire hazard. I put a handful of insulation under it to pad it in case it is vibrating against the framing. But only time will tell.
I suspect you have snow right now, so going on the roof isn’t really an option. However, you may want to take a look (at least from the ground) at the furnace exhaust and make sure there isn’t a bird’s nest up there. It’s a long shot, but it’s possible that there’s birds/animals in or around the flue that are scattering when the furnace first kicks. If that’s the case, they may just not be present all the time.
While the cards in a bike spoke noise points me in the direction of something spinning, likely the inducer motor based on when you said it happens, I’m working on the location of the noise. OTOH, noise does travel, so there’s that.
As someone else said, you may want to pop the inducer out and take a look. Check for debris or a broken blade. The exact spot, what it looks like and how easy it is (generally, not that hard) to get out will vary from one model to the next. Also, if you’re comfortable, you can apply voltage directly to it so that it spins up while you’re standing right next to it. Then you have control over it. You can listen for sound right there, you can run upstairs and listen for sound there. You can turn it off and try again later (without doing anything else at all, if the t-stat calls for heat, the furnace won’t fire if the inducer doesn’t spin up first, so that won’t mess with your testing).
Also, are you totally sure that the noise happens as the furnace is turning on? I only ask so that you aren’t spending time chasing a bad lead. I believe you, don’t get me wrong, just make sure you never hear it at any time other than when the furnace is running (the inducer does run during the (almost) the entire time the furnace is on).
One last thing, I wouldn’t rule out something like vibration or rattling, but it’s likely not thermal expansion since when the inducer first turns on it’s just blowing out room temperature air. It may be a bit warmer than the air in the pipe, but it’s not air that’s so hot it’s going to make the pipe expand rapidly enough to sound like that, at least I wouldn’t think so.
It’s been a couple of days and so far I have heard no further noise. Could be a coincidence but I suspect that there is vibration in the duct that was rattling the sheet metal against the framing. Why the duct would be vibrating in the first place I don’t know. I am leaving the ladder up for a few more days just in case
Still no noise. Here is what the sheet metal looks like after I stuffed a little loose insulation under it for padding to prevent it from contacting the framing.
I still don’t know the root cause but it doesn’t wake me up anymore.
but it doesn’t wake me up anymore. 