I think that the first thing I'd try is to "un-biamp" the speakers, and run them on the other amp since apparently the rest of the drivers aren't buzzing. That, or swap the amps and run the tweeters only on the second amp. If the buzzing goes away using the other amp I'd look for a ground fault with the tweeter amp.. If the buzzing doesn't go away, it's most likely the tweeters themselves are possibly "blown", or maybe even some kind of mechanical problem, i.e. a grill or mount buzzing.. How low or high a frequency (relatively speaking!) is the noise?
Best of luck!