RE: new mystery
Assuming it is submersed and no air is getting to it I would look at the impeller for imperfections. Build up or any small damage might cause it. The pump in my nano did the same thing, I pulled the pump out and cleaned it. While doing that I found a small crack on one of the impeller blades. Heat inside the pump might also cause it. I could be way off, but you never know.
John
"Critics are men who watch a battle from a high place and then come down and shoot the survivors." Ernest Hemingway