New Rheem 90+ 2 stage furnace install (the A/C coil is not installed yet), the blower motor is making a "thruming" noise, changing motor pitch and noise volume slightly with about a 2 second period. Large supply and return ductwork with low static pressure (about .1 inches on the supply and .2 on the return). Unfortunately, the noise cycling is quite annoying in the basement where it's installed, and the noise travels up the returns into the living room. I've done some noise reduction work in the ductwork which reduced the overall volume, but the constant changes in the noise tone is not tolerable.
Someone has suggested that it's due to the load on the motor varying, but they had no thoughts as to why the load is changing.
I am going to have it checked out by a pro (the original installer is AWOL, a long story I won't tell here), but I was trying to get some additional thoughts ahead of time as to what could be causing it, what additional diagnostics would be useful, and what possible solutions were.
I've seen where some type of debris got sucked into the blower (even a piece of paper or a hand rag) and it throws the delicate balance of the fan out-a-whack. Maybe the motor mounts are messed up, too.
Try increasing the static slightly by covering 1/4-1/3 of your filter with cardboard as a TEMPORARY test,(Do not leave it this way). See if the added static makes the noise go away, If it does, then maybe it will go away when you add the coil.