I take pretty good care of them, I clean it out once a week or so. These no matter how well I cleaned they and replaced parts continued to make nosieI found (especially with sand) I had to clean mine almost every other day.
I normally rinse the sponge and shake out the media in some water change watr, at the same time, and simply rinse out the chamber where the impeller sits, occasionally use an old tooth brush to make sure there isn't anything still there.
Agree totally with the one grain of sand scenario, its amazing how finicky they can be.