ARghh no dont panic !
There may be a FAQ about this, BTW. (hint hint, Joe)
There are NO bearings on your pump, trust me.
THe rattling sound you can hear comes from some kind of protection on the impeller against little rocks and solid things you may find inside an aquarium...
Empty your system and take out the pump. Open the impeller chamber.
See that little propeller-like thing ? It can turn relatively to its shaft. There's a pin to limit rotation though (so it flow is not interrupted in the event of a solid thing coming in). This produces the sound you can hear.
Glue the propeller to the shaft. Make sure NO GLUE drops *into* the central part of the shaft (where it's mounted on the axis) or you'll end up with a dead pump.... Cyanolitic or waterproof epoxy can do it. I used Cyano (Super Glue 3) with success.
You'll see, no more noise...