Things that go bump in the night

We have this new furnace that works WONDerfully! However, there is something different about it that has caused Gail's imagination to run wild. It seems the chambers in this new furnace are a bit larger than the previous one, so the pressure and the heat expansion causes the walls to "Thunk". We mostly hear the metallic "Thud" when the furnace turns OFF. It's just the metal chamber wall bending back into place when the heat and pressure are off. So this is how Gail's mind works:
When the furnace is running and blowing, there is a lot of air pressure going through there. Then the air pressure goes off. Next, she hears a "Thud." In her mind's eye, she is picturing some small, trapped rodent or a squirrel or something, kept suspended and spinning in the air while the air is blowing, but as soon as the air shuts off-- THUNK! "There goes that small animal again!" What an imagination. Silly girl. So of course, now we ALL think about it each time the furnace goes off. Sometime soon I believe Mike will get down there and screw a supporting metal strip across that wall in the chamber to keep it from thunking with the expansion and contraction. It thunks when it first heats up, too. We just don't hear it because of the noise of the fan. Someday. In the mean time, we just cringe a little, wondering if there really IS some small, trapped creature living in our duct work! ???!!! [not really, but it makes for a fun story] ;-)