Water

Clogged evaporator drain? Get under car and shoot some compressed air up the drain tube and see if water comes pouring out. Are you getting water on your front floorboards? Is the sound worse while using AC? Do you have water draining on ground after using AC? ...say in a lot or on your driveway right after you parked?

Or possibly a leaking heater core and the coolant is pooling inside the ductwork and sloshing around when you drive?? Check your car's coolant level and check the carpet, especially on the passenger side to see if its wet.

It happened on my truck ; it turned out one of the doors outlet was jammed shut and rain filled the inside of the door . Shake your doors while standing beside it.