On Tuesday, March 5, 2019 at 8:35:13 PM UTC-5, wrote:
I have a similar problem. When the furnace shuts off, I've been getting a squeak. I took the access cover off, and don't get the squeak when the cover is off. When I put the cover back on, it squeaks.

I'm thinking I may not be getting adequate combustion air and there's a sudden change in air pressure in the furnace when the burners/fan turn off.

I tried putting some silicone spray lube on the rubber gaskets on the access panel, but that hasn't solved it.

I'm not sure how to go about checking the combustion air supply.

Thanks.

First question is what do you mean by the "furnace shutting off"?
If it's when the burner stops, then it could be something to do with
the combustion air supply, which presumably is via fan. Or is it
when the air handler blower stops, in which case it's not combustion
related? If it's the latter, could be some duct work at the furnace
flexing as the air pressure stops.