Try putting a "weight" on top of your 500. Just make sure you put a thick towel on your 500 before doing so. I've since put a towel under mine since the beaver was shuffling around too much.
I have the beaver (about 15lbs) shown below on top of my 600. On really demanding scenes like the depth charge scene in U-571 or the pod emerging scene in WOTW, the beaver prevents the "rattling" resonance that I hear.
The 600 (and I imagine the 500) are not infallible. If you put on tracks 6 or 7 of Plastikman: Closer, you will hear what some that have heard my 600 describe as "basket noises". However, I tried playing the same tracks on a Hsu VTF-3 and not only was half the bass missing, the chuffing noise out of the twin ports was awful. And my Sony was a different tale all together as I could smell the amp cooking and could hear the driver's motor structure bottoming out.
Finally, make sure that it isn't your room (or a window) rattling.
