after the furnbace runs it goes clunk , then about a min and half later it goes clunk again . this happens about three times . what is the clunk? thansk

Is this a gas furnace? Can you see the flame turning on and then going out?

I have a similar issue with my natural gas furnace. It is a 8 month old new build house with Rheem Classic Series: Up to 95% AFUE Single-Stage PSC Motor gas furnace. After is cycles and turns off there is a loud bang anywhere from 30 seconds to 4 minutes afterwards. There is also a consistant clicking noise that is most pronounced at end of the duct about a 20 foot straight run from the furnace. It can be heard in the family room and a second floor bedroom vents. The HVAC installers came determined that it was "oilcanning". He cut two square holes and repaired the holes but the noise continues.

Any advice on how I can stop the noise? Can and should the HVAC company be held accountable. I do not want to get the "you just have to deal" attitude since it is a new build.

TIA...Ann Marie

My Furnace:

Rheem Classic Plus - Super Quiet 80, gas furnace; heat only - heats my tiny house wonderfully (775 sq ft) no AC; 3 yrs old.

I've been very happy with this furnace but it makes a sound as it finishes a heating cycle

The sound is a single dull metallic "clunk" (imagine lightly taping an empty tin coffee can with a small wrench).

I thought maybe it was a noisy valve.
My furnace is located in the attic, so I climbed up, pulled off the door and watched and listened.
I was right next to the gas valves, so I know I would have heard if the sound came from them. As the heat rose the thermostat in the room below me told the furnace it could stop heating. So, the gas supply was stopped (silently) and the flame - of course - also when out.
Then, almost the moment - maybe a half-second after the flame went out - I heard a faint "clunk" - coming perhaps from the vicinity of the heat exchanger.

Okay?

So this is my theory - and I'd be interested very much in some feedback:

I think it's the sound of contracting metal.

Being in the attic, part of the return air comes from the very cold attic itself (as well as from the return vents in my small house).

So I think the sound is caused by the very cool air coming in contact with very hot metal (heat exchanger, etc) and, since the flame has gone out, there is a sudden contraction - results in a sound (that single rap sound of a small wrench on a tin can).

The other thing I noticed is that sitting in my attic - quite close to the furnace - listening - the "clunk" was quite faint. So, seems to me it is probably a minor sound (not loud), but the ductwork seems to amplify the volume of it; sounds worse than it is - and probably nothing to be concerned about?? Or is it? I'm a poor DIY type; can't afford to have someone make an unnecessary service call and as we have seen - some of these service techs don't really know; perform unnecessary work, etc.

Come to think of if - and you with the same problem make your own observation - it seems to me that the clunk is the louder when the outside temp is the coldest - which might re-enforce my theory about the sound being caused by cooling / contracting metal.

Like I say - the furnace is working great and I think the sound, in my case, is nothing to be alarmed about.

Thoughts??

Actually, while you all are mulling this over, it occurred to me that I may actually be able to tell my furnace not to run the fan that I guess is designed to take advantage of the little remaining heat off the heat exchanger - immediately after the flame goes out.

I will also add that that post-flame blow - blows cool air into my house - so seems kind of counter productive anyway - yes?

I'll have to look at the manual, but I think I can set a pin jumper on the furnace control board so the fan stops at the same time the flame goes out.

Before I do that - it would be nice to hear what anyone thinks about my above theory.

thank you