Furnace makes squeaking/rattling noise after burner shuts off

Red Squirrel · 09-15-2010, 12:31 PM

I've been wanting to get my furnace professionally inspected anyway (hard to find people at this time of year, still waiting for phone calls), but it just so happens that it started doing this not too long ago.

Basically, it runs fine, but when the thermostat calls it to turn off, and the burner shuts off, there is a squeaky rattling noise. It sounds like it's coming from the heat exchanger area and not the blower. I'm thinking there is something loose somewhere that with heat contraction, moves and the air passing by is making it shake.

I don't think the heat exchanger is cracked or anything as my CO detector probably would have picked up on that, or I'd be feeling dizzy or be talking with Jesus.

The sound does not seem to occur during normal operation of the furnace, or if I make the blower run on it's own, did it all summer and was fine. It's when the blower is running after the furnace has ran. It's also random. I only really use the furnace in the morning and maybe at my lunch, but it seems it do it after it's ran for long enough. If I turn it on, let it run for 5 minutes then turn it off, it wont do it.

Has anyone ever heard of this?

hvaclover · 09-15-2010, 01:14 PM

Could be the ht ex just contracting or the blower making noise as it winds down to stop.

how old is it? belt or direct drive blower?

Red Squirrel · 09-15-2010, 04:25 PM

I believe it was installed in 2002. It's a Bryand high efficiency. The blower is direct drive. Don't think it has any different speeds but I may be wrong.

hvaclover · 09-15-2010, 04:45 PM

It is either a multi speed PSC or variable speed.

pull the blower cover and look for loose motor mounts. Could be the motor shaft has a bearing going.

Also with the furnace turned off, check the inside of the blower for paper or other foreign matter .

Also turn the furnace on and lit it fire the burners and blower. wait for the cycle to satisfy the stat, see if the furnace vibrates as the blower shuts down.

Red Squirrel · 09-18-2010, 01:39 PM

This is kinda silly, I ended up getting a service guy out as I figured it would not hurt for him to overview the whole system to make sure it's in good shape overall. He took it all apart and spent a good 2 hours or so on it, I showed him a recording of the sound and he was stumped. It was never doing it when he was there and the blower panel was open.

Turns out, the new filter I put in is too thick and there's not enough air flow so the motor is working harder, and vibrates due to the higher negative air pressure inside. He told me those are great filters, but problem is lot of systems don't like them. So I'll have to go buy one of the cheaper ones that are just fibers.

kenmac · 09-18-2010, 02:05 PM

I figured he would find some of those nuts you've been hiding for winter

Alot of people go with those ''better filters'' & end up starving the blower for air

**beenthere** · 09-18-2010, 02:07 PM

Was it a 3M filter?

ssms · 11-10-2011, 12:16 PM

Just to add my 2 cents worth - I recently had a Rheem furnace installed and am getting the same sounds when the burners cut off. They aren't coming from the blower compartment, but from the heat exchanger. The installer told me the exchanger gets so hot that when the burners do shut off the sudden cool down causes the exchanger to contract, and that's what I'm hearing. Said it should eventually go away on its own. He even went ahead and changed out the exchanger with a different one, and I didn't hear anything the first few times the furnace was run, but the sound has since returned. Maybe it has to do with the in-shot burners vs. the ribbon style, which is what my old furnace had.

how · 11-10-2011, 08:35 PM

Hey red squirrell
I was thinking that if it was just negative air pressure from a too restrictive air filter, it would of made the same sound whether the furnace was running for a short time or a long time. I would think that the added heat factor of the restrictive air flow was allowing something to expand & rattle on cool down or failing bearings to squeal when the furnace had gotten hotter than normal during a longer run time.
2 hours to track down a restrictive air filter. Glad that wasn't my call.

ben's plumbing · 11-10-2011, 08:37 PM

maybe it the furnace is not sized right for the duct work or the ductwork not sized right for furnace causing over heating of heat exchanger.....is the furnace cycling during single call for heat....

ssms · 11-12-2011, 11:39 PM

Ben's plumbing,
Not sure if you were asking me or Red Squirrel. I have the same sized hvac system as before, 2.5 ton, and based on the square footage of my house that should be correct. I didn't mean to imply the exchanger gets hotter than it should, but that it gets hot enough to cause the rattle noise I'm hearing (which only lasts for a few seconds) once the burners cut off and the exchanger contracts, per the service tech. I just figured it had something to do with the different type of burners from what my old furnace had. But what is strange is the owner of the company came out and said he didn't know what was causing it. Then his service technicians came out and said it was the heat exchanger contracting. So who knows. There's next to nothing on the internet from what I've been able to find. When they replaced the exchanger I didn't hear this rattle the first few times the burners cycled off, then it returned. I just hope when they said it'll eventually go away, it will.