Have a Carrier gas furnace, model CNRVP3014ATAABAA, that is supplying
a two story home (800 down / 1200 up) with respective air return sizes of 14x14 and 20x20.
The owner complained of excessive noise from the air return…which I agree is loud.
The unit is housed in the laundry romm on the second floor with the return going directly to an adjacent wall that separates the laundry room and hallway.
The 20x20 air return is the only one upstairs that has to take care of 3 bedrooms plus the addtional rooms up there.
While I am no HVAC contractor it seems like this was a bad design.