I think I have a similar problem and my guess is, itÂ's not multiplayer related.
It does not happen regularly, but it seems to come up whenever I am in a chopper or close to a heavy automatic weapon like shilka, vulcan, M2 or the Autocannon of the Apache.
If your buddy can confirm this, try a new soundcard driver.
BTW: My soundcard: Creative (Duh) live player 5.1
Coyote