This feedback produced by my new MacBook Pro is a little less harsh. I attempted to control the way it was feeding back by moving a cupped hand up and down just above the built-in mic hidden under the left hand speaker grill. The closer you get to the cupping the mic the louder the feedback is. Actually I wasn’t sure where the mic was when I started this experiment and used my cupped hand to locate it. By the way, cupping the mic is generally a bad idea. I hope it doesn’t void my warrantee.

Cupped Hand Controlled Feedback