Sand will absolutly kill the impeller. I had a 350 on a 55g and had the down tube to close to the bottom. As a result (aided by Africans) the sand was sucked up and in. Once the impeller is "scored" its toast, IMO.
I found that placing a spacer under the impeller(a plastic bead my little daughter gave me from her craft box...=) helped quiet it down. Just run the bead onto the impeller shaft. It helps keep the impeller from "bouncing."