I have two HVAC units in my house and the smaller of the two is in a hallway closet next to the living room. It brings in air from a vent below the closet door. When the system is running the noise from the fan and air coming into the vent is pretty loud. It seems like most of the noise is from the air coming in. It is loud enough that we find ourselves turning up the tv a bit when the fan kicks on.
I am thinking about adding a return air duct and blocking off the vent below the door. There is plenty of room in the closet to bring the new duct in and use the newly blocked off area below the unit as an air handler box. I would add a new return air vent or two to the ceiling somewhere else.
Does this idea sound ok to the experts out there or am I off on my thinking?
I tried to sketch up an illustration below to help visualize what I'm describing.
As long as the return duct is properly sized you won't have any troubles with it. You'd have to determine how much air you need back in the return, and size the duct for that - don't base the duct size on the space you have available, it must be the right size for the system or you'd just be causing more troubles.