I believe the empeg's (RioCar in my case) metal body (the box containing the electronics) is happier (at least in my situation) when also properly grounded through the metal shell of the sled.

The sled shell itself must be properly grounded, AND the metal bumps on the inside of the shell must make decent contact with the empeg's casing. I have found that if wear, corrosion, or dirt have allowed the empeg to be inserted without the casing being in solid electrical contact with the shell around it, then I will hear electrical noise (hard disk accessing, general buzzing).

When this was happening to me, jiggling the unit would sometimes reduce the noise, as would some re-insertions. Adjusting the pressure points inside my (now well grounded) sled to increase contact pressure eliminated all the noise.

I discovered this after spending time with a decent digital meter looking for ground problems, and not finding anything of significance at the sled docking connector. I then thought about electrical paths other than the docking connector, which led to me looking at the ground path for the body of the unit.

This may or may not be a factor in your case, but I think it is something to keep in mind.