We replaced all of our ductwork and now have a knock in the floor. it is not expansion or contraction as it is not a metal noise. it only happens with the furnace, not the AC. we have had HVAC people out and flooring people. we have opened the ceiling where we hear it and can’t see anything. it happens totally randomly without any obvious weather pattern. it can happen once or 4 times in a row and then not happen for a week. there is no pattern to the noise and it can happen at different volumes. I have a recording of it.