a lot of the time when this happen is when you take the pump cover apart to clean. when you put it back, there's a black rubber in the cover that hold the ceramic shaft to the center is probably not at the center. when that happen, it push the shaft to the side and cause the impeller to make noise. I believe this is that happened to your case. This is a very common problem when the cover is not put back properly. Glad you got it figured it out, don't forget to post some pictures now of your nasty skimmate .