Hmm, my magnum 350 was loud as hell also. I noticed it was louder after I dropped it and it was already loud before that. But it would make this rattling sound like there's something causing the rattling sound but there was nothing, no dirt, sand or anything. My tank was barebottom and I was running the micron cartridges or the carbon container with one of those blue bonded filter pads instead or carbon inside. If ur impeller looks good then maybe it could be coming from the motor?