I have a magnum 350 and i love it to, i took the carbon basket and filled it with floss, and put in pot scrubbies instead of that blue foam, works waaaaaaaaay better... but to your issue, if my impeller makes that noise (which it does after almost every cleaning) i tear it all apart and clean inside where the impeller sits very, very well. After that it's silent, also, try shutting it off and shaking it back and forth to help release the excess air inside the canister.