I remember seeing a thread about others Fluval 406 making a rattling noise as if the impeller was damaged or for other reasons. Well I recently reset up my 55 gal and I started getting that noise also. My mystery has been solved though as my tank is in the living room and 9 out of 10 times when I use my microwave oven in the kitchen I get the impeller rattling. It stops when the microwave goes off. So my guess is the electrical wiring used by the microwave and my power strip for my tank are on the same circuit and when there is not enough juice in the lines the impeller slows creating my rattle. I think others mentioned they bought a different brand and there had no issues. Maybe the 406 uses a different amount of electricity compared to other brands. Hopefully this may help others.