I had the same problem with one of my M60's when I first got them. Listening to the Matrix Reloaded Soundtrack (disc 2) it made a buzzing sound only in one part of the 4th track at about 90db. Kinda sounded like aluminum vibrating on aluminum, know what I mean? Anyway I remember having to tighten the screws down pretty tight in order for it to go away, and now when I play them even louder than I did before (about 105db) they don't make that sound at all. That's my story.
EDIT- BTW, what scene in POTC does your speaker make this sound?
