Are you sure it's the empeg and the sled vibrating against each other?

How solidly is the sled mounted to the frame of the car? Maybe it's the sled rattling against the frame of the car?

Or perhaps it's a rattling handle instead?

If it's really the case and the sled, the sled has bumps which are supposed to slide against the player casing. Thse bumps are on tabs. I'm sure you can bend these tabs so that they're tighter.