Based on the mfr's pic and description, I'm guessing that the "sub"-woofer is using the entire volume of the studded section of wall its mounted in... since farting requires the passing of gas, have your friend try plugging the port(s).

...the paper on the insulation in the wall may be vibrating against the drywall, making a funny sound as well.

just my initial thoughts.
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Scott

My HT