Goodman furnace making flapping noise in defrost mode

[roger2389]

When my furnace is in defrost mode (fan not turning outside) I hear a loud noise that sounds like there is something flapping underneath the house. I have been under the house and checked all the duct work and found nothing loose. 2 different companies have come out to check it and of course it want make the noise when they are here. Odd thing is it does not make the noise everyday just usually early in the morning when I am trying to sleep. Anyone have ANY ideas on what could be causing this noise?

[Freon]

When your heat pump is in defrost mode, does the indoor air circulate in the duct work? Check the copper tubing and see if it runs under the house where the noise seems to come from. If it does, next time you think there will be noise, put your hand on each tube CAREFULLY and SLOWLY. Feel for vibrations and possible pulsing... a burst of bubbles flowing through the tubing. Check the suspension of the tubing.

If the indoor air does continue to flow then usually the furnace will come on. Possibly the hot air from the furnace causes duct expansion (metal ducting) and you hear that noise.

[MTI STUDENT]

are you using a timed thermostat with a low night setting?

[heatseeker]

Maybe there is a wild animal under there that gets scared when it goes into defrost. Second guess liquid line pulsating.

[heatseeker]

Third guess there is a duck caught in the duct.