I have an MBP and my girlfriend has a MB. Whenever we are video iChatting with each other, we get a very strange audio feedback issue. I will try to explain it, and perhaps someone may know how to help? Anyway, the sound is like a submarine sonar beeping as it passes over an object. But the beep emits 4 times a second and is incessant. There is no consistent rhythm. The volume of the beep fluctuates and sometimes gets so loud that I can't hear a word the other person is saying. I've tried to experiment with the sound. If I whistle at the same or very similar frequency, the sound gets extremely loud, drowning out all other audio. It then slowly dies down to a moderate level. If I cover the left speaker grill with my hand, it also gets extremely loud. Lowering my volume helps dramatically, but the problem persists. Is my microphone broken? Does this have to do with some noise cancellation software?