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?

Thanks in advance