The speaker and the microphone are only about an inch apart. When I first got my M200 years ago I got bit by the feedback issue a few times before I realize that somehow I was inadvertently spinning the volume thumbwheel to full blast. Upon reboot it would squeal something fierce. Very embarrassing.
My guess for your issue would be that perhaps the rheostat in the volume wheel is having issues. My current gets crackly and imprroves slightly if I spin it back and forth a few times. There used to be a cleaner/lubricating spray that could be applied to revive old components. If that doesn't work it may be time to shop for donor organs.