Went for a shower.. Came back... to my aquaclear 20 impeller making the most horrific noise ever.... EVER!!!! Its normally pretty silent... So I was sort of horrified it could make this noise in about 5 minutes... It's been running for about 4 months with no problems... I took it apart and washed everything.... Looked for sand... There wasnt any (my substrate is fine sand, but I have a sponge over the intake) It has quieted down a little since cleaning it, but theres still this horrifying noise coming from it... The worst part is that it stops for a couple seconds every minute or so... It sounds completely normal for about 30 seconds every couple minutes.... any idea's whats going on?