Came across a problem I didn't have before replacing old furnace. Nor did i have this problem when it was first installed. I've noticed the ducts banging when the furnace stops the fan cycle I have set. We are in ac mode and I let the fan come on every 10 minutes just to keep the air circulating (new thermostats allow some of the neatest things). I noticed that when the fan shuts down I hear a very distinctive pop from the return plenum (vertical section going into furnace filter then furnace blower). I then noticed the supply plenum shrink (for want of a better term) back in, like it is deflating, followed by another pop but much lighter than the first. My guess is that it is a pressure question and not a heat/cold thing as only the fan is running. I do however, notice the same kind of banging when the ac is running and stops. I use the more expensive brand air filters and the filter is clean. I checked the supply at the furnace and there isn't anything blocking the way. I imagine it could be something further up the supply line. Returns are not blocked and mostly all registers are open. I keep basement ones closed during ac season. Anyway, I've taken up way too much of your time asking you to read all this but I was wondering if anyone has any ideas as to what could be going on. Newly installed furnace that didn't have a problem then, all of a sudden, it starts popping. Thanks

[left]I have this new furnace and all the measure are between the old furnace and new one is good except for the height of the new furnace..it is 3 3/4 inches shorter... So it looks like I will need a small piece made ...in order to connect the plenum. Should I have the new piece made so when it joines to the old plenum , it fits on the outside or the inside ..Is there such of a joint where the old would fit into a flange ...on the new one ...if that’s the case, the piece would have to be an exact measure.. if if I just fit the new piece up inside the old plenum ! How much of an overlap should I do..maybe a couple inches....thanks for info. [/left]