As HP says, the NIC (Standabkopplung) will affect the idle load and hence speed in different states.
On my 535d, at idle with NIC happening, I got a nasty resonance. A little tracking down found this to be the long air intake pipe. I ran my finger up and down it and could easily feel the point of maximum vibration and cured the issue entirely by installing a custom resonance-tuned damping system to the intake (OK, I tie-wrapped it to a solid point). Works a treat, now completely silent.
