That is very strange, I have been emailing with Phil as well about what it could be. Does the noise stop if you submerge the skimmer a little more. Have you tried pushing in the metal shaft and pushing down on the drive unit all the way to make sure everything is seated? The motor has no moving parts, so the possibilities are pretty limited to basically a bent or loose shaft or some protrusion in the motor or impeller housing it is rubbing on, or being starved for water.
