A couple weeks ago my Emperor 400 had to be reprimed after a water change (the level dropped too low). When I started it back up it was really loud. My friend and I took it apart thinking maybe the impeller was damaged. Nothing was wrong with it. I have an identical filter on the other side and its perfectly quiet. Its almost a grinding noise. Anyone familiar with this?

The impeller is alinged correct and the impeller housing seated
right? Most of the time that's the culprit. Are the rubber pads
still on that rest against the glass in the back? Anything
sitting on top of the filter? Is the flow control all the way up
and running correct? Are the bio wheels alinged right? Is
there anything stuck in the impeller housing?

Just some of the normals thoughts for odd noises.

Oh am I familiar with that!!!!! I remember when I bought it....I was expecting sooooo much from it,& when I primed & started it-that's exactly how it sounded! :crp I played with that thing off and on for a week.Nearly drove me crazy!!!!! It finally stopped one day.I just constantly wiggled,pushed down on parts etc. Since then it has stopped twice on me because of doing water changes,& I went thru the same thing! Believe me......Now when I do water changes,I make sure I never go too low! I can't tell you why or how exactly-I just kept messing with it! And by the way,for anyone wondering....when you do get it right,it's the best!

Last time mine made horrid noises was because in my futzing around with it, I managed to move a bubble bar such that the bubbles were sneaking up into the intake -

Paul's suggestions are all good, too.

is that a bad thing? will it somehow mess up the filter, or is it just annoying?

Oh,it's annoying allright!!!!

Heh, Marineland.

Btw, has anyone tried their new Penguin series?

no, but I was thinking of trying the 350B

hehe...that it is! I've got one happening like that right now...right next to my bed. But it is only temporary.....just was wondering if it was in any way bad for the filter or anything like that.

I had the same problom with mine, but the reason why mine made sounds was the water flow is set on low. Wish they can post that in the instructions!