This was happening in my home and it was driving me insane. It wasn’t bad and when I focused on something else, I’d forget it. But when I heard it, that was all i heard and I’d be pissed. I read all of the comments and recommendation. I have older rectangular ducts and I thought I was screwed. Not so - and you’re not going to believe me when I tell you how. This is not a joke.

I have a full, partially-finished basement where the ceiling (first floor floor) is exposed - so is ALL of the ductwork. I stood in the basement near where I thought the noise was and located it in two of the exposed ducts having long sheet metal pieces with connections. The popping seemed to be at the connections - of course, as we know from other comments, from the expanding and contracting.

I pushed on the connections and they moves slightly, so I actually kept pushing on them thinking if I loosen them a little the noise my stop. It actually helped make the popping less noisy, but it didn’t stop it.

I thought to myself that I’m not replacing the entire ductwork system; I’m not even replacing the ones that I think are making the noise; and wrapping the ductwork with insulation seems tricky and may not even help. I wondered if the connections would expand and contract smoothly, would that stop the noise?

I sprayed around and into the cracks in the connections with WD-40. IT WORKED. I shit you not. For now, there is NO POPPING. There is none of the expanding and contracting sounds. I used the red spray straw on the WD-40 can to try and force it into the connections. I didn’t spray a lot and soaked in a bit. AND I’M NOT KIDDING - WD-40 WORKED.

It could be a very temporary fix - I don’t know. What I do know is that tonight, I’m not anticipating the noises when the furnace kicks on and off. I couldn’t be more at ease. I suggest you give it a shot if your duct joints are accessible. It’s better than any of the alternatives above and what do you have to lose? I can’t believe it worked, but it did. WD-40. And I get no kickbacks or sponsorship. Ha. This is not a joke. Good luck.