Sounds to me like the impellor might be worn. They'll rattle around pretty good in there if it is. Eventually it will lock up and not pump water. If you rap it good does it stop for a second? That might tell you if it's worn. The other thing is they don't have a bunch of power. So if you get a stone in it, sometimes they'll rattle around in there. That will do some damage to the impellor too. You'll want to take it apart. If you turn the filter over I believe the pump part just untwists from down there then pulls out. Be gentle though cuz it may unscrew too. Can't remember. In any event, it seems to be that some impellors are hard to come by. Might be the end of it.

gpshemi