Does the problem persist when your AC adapter is unplugged?
What happens when you use a different mic? Does the screeching go away when you cover the mic with your had?
The screeching is probably what's known as a feedback loop. It occurs when your mic hears what your speakers are playing, which is what the mic hears, which is what the speakers are playing...this loop goes on and on, getting amplified by your amplifier every time, which turns the audio into a screeching sound.