My furnace makes almost a siren sound when it shuts off, I can hear it through my return. What is this and how do I fix it?

P.S. It also makes the big bang sound at the end of the cycle but I've read a bit about that.

What you describe sounds like an air leak of some type, or a motor that is approaching dry-seize failure. My "big bang theory" (I just couldn't resist) is duct warping.

Should I replace the motor?? Is that hard to do?

Well, confirming the source of the noise should come first. You haven't mentioned a brand/model # so I have no idea what kind of furnace you have.

It's a Rheem Criterion II Gas Furnace Model RCBA-4882GG24. The sound happens at the beginning of the cycle and at the end now. It sounds like a fan, but maybe not. I first heard it in through my return upstairs.

Thanks.

John

I would observe from closer range, to pinpoint the source of the noise.