You ever hear the brakes on a large aircraft that’s taxiing slowly? A high-pitched ‘hoooohoooo’? Or maybe a truck or something has the same sound. Well, that’s the sound my heater just made as the thermostat kicked on and it started making heat.
The propane guy came today and refilled the tank. The tank has been filled several times in the two years I’ve lived here, but this is the first time I’ve heard this sound.
Any idea what’s causing it?
Could be a loose fan belt on the blower, or the fan motor needing lubrication.
I’ll second a belt being a possible cause. There aren’t many moving parts in a furnace. And that which doesn’t move, make no noise. Or so Buddah might say.
Have you had the furnace inspected/cleaned since you moved in? We have a gas furnace and maintenance is always a good idea for efficiency. Plus, using a fuel that offers the bonus those electric heat wimps don’t deal with (your house blowing up) it’s never bad to have it checked out. Hope we aren’t reading about you on Fark soon.
Seriously, have it looked at. It might just be a belt. But it could also be a hamster plotting your death. See if Cecil is free this weekend. 
I guess I’ll third the loose belt idea; the description sounds like a car’s fan belt when it’s loose, which is about the worst noise in the world to my ears.*
*It’s actually a tie between that and someone saying, “We need to talk.”
Doesn’t appear to be the fan. It just did it again. The sound happened when the heater started to heat – i.e., when the gas first started to burn.
If it’s not the fan belt or motor, I’m stumped. I’m assuming your propane furnace works the same way as a natural gas forced air furnace. If that’s the case, the only difference is that my furnace is supplied by a natural gas line and yours by a propane tank. Is there some way of checking the pressure to make sure the gas supply is smooth and adequate? Perhaps looking at the burners to make sure they are all lit? If there is some kind of buffeting occurring then the noises could be coming from there. It wouldn’t hurt to call the furnace people and ask, those guys have seen it all.
Or possibly a Corrugaphone.
I took the front cover off. Coincidentally, the sound came again. Couldn’t tell where it was coming from. I did find the pilot light control though. That will be usefull when Winter is over. I think I have a manual around here somewhere. I’ll have to find it when I have time to see what I can see.
I wonder if I got some ‘bad gas’? It was 32ºF this morning, and it was pretty cool last night. If there is excessive moisture in the gas, perhaps it is freezing? If so, it might restrict the orifice and make it ‘whistle’. Corellation does not equal causation, but it’s interesting that this nevert happened until I got the most recent tank of gas.
Certain wind directions and speeds will make a furnace ‘whooaoa’ at you.
A too full and cold tank will cause all kinds of problems. Ever notice that hot air balloons put warming blankets on their tanks?
Actually though, I would look to see if the airflow could have changes suddenly or maybe it is ice in the line and you are getting a variable flame, they can actually almost vibrate or shudder under some conditions.
Get your gas man back and have him listen and check.
Don’t forget, propane is heavy and can really pool up and hurt you if you have a leak.
Does it just do it on flame start or all the way through that heat cycle?
Can you feel an actual vibration?
Get it checked…
Doesn’t happen all the time.
It only makes the sound on flame start.
Haven’t felt a vibration.
Have someone stand at the thermostat and turn it up and down making your flame go off and on…
Also, open the door and whatnot like you would on a gas hot water heater and get where you can see both the pilot light and the main burner. May need a mirror. Now cycle the furnace and see what is actually happening.