Sounds like chuffing. That means your sub is being pushed too hard. I would imagine it's fine, but you don't want to run it that way all the time.
The problem is that your room is way too big. An 8" sub is generally a good choice for a small room (such as a bedroom). For 4500 cubic feet, you need a 12" sub. Might want to do some research in the AVS subwoofer forum.