"blueman" wrote in message
...
The sheet metal return plenum on one of our 2 otherwise identical gas
furnaces gives out a loud "bang" as the duct contracts inwards when the
blower starts and again when the duct returns to its original
conformation when the blower stops.

The other furnace that doesn't "bang" seems to have a relief "X" on its
broad surface while the one that does "bang" doesn't have the "X".

Well there you go. Those Xs are there for a reason. Get the installer back
to do the job right. Otherwise, you'll have to add some external
stiffening.