So heres the scenario:

engine warm or cold
hvac on
a/c on or off
outside temp makes no difference
blower speed makes no difference

The problem:

At very light throttle openings or closed throttle decel, between 900 and 1200rpm my 2005 S500 4matic makes a bit of a groaning/grinding noise. It is not terribly loud, but can be heard plain as day with the radio off. It can also be felt through the floor board if your left foot is flat on the floor. It can be felt through the gas pedal, but not as easily. If you turn off the hvac unit entirely, the noise goes away completely and does not return.

The HVAC control unit was recently replaced, with no change in the noise, and HVAC function otherwise is perfect.

At any other rpms or under heavier throttle, the noise is not apparently and can not be felt.

So, considering this is related to engine rpm, I figured it may be related to the compressor (or compressor clutch). The dealer figured it may be a slightly noisey a/c purge valve, but I don't understand how that would create such a noise.

Do any MB techs have any ideas about this noise?

I've looked at EPC, and it doesn't look like anything HVAC related runs under the drivers side floorboard.

Thanks for any help.

sounds like it is air in the heater core.

Remove coolant cap, warm up the engine and turn heat on full. Rev up the engine and try to get any air out of the system, top up the coolant level.

interesting assesment.

From my experience with mustangs, air in the cooling system would cause overheating, of which I have none.

Furthermore, wouldn't that noise still occur even if the HVAC unit was off?

Thanks for the help though.

ttt, no other opinions?

thanks

some additional help here would be great.

thanks.

You should have the A/C system evacuated and re-charged. Sounds like it's low on freon and your hearing the expansion valve working. You might also have an A/C line rubbing on the frame rail or shield causing your noise. Engine mount collapsed can also cause extra noise or vibrations felt when A/C is on. Hope this helps.

thanks.

weird thing is, the noise doesn't go away when i hit the a/c off button, only when I turn the whole unit off.

I'll look into the expansion/recharge.

Any idea on what this runs at the dealer? Few hundred I imagine?

thanks.

Anybody know if you can get the compressor clutch bearing separately from the compressor?

Motor mounts have been known to show up as vibration in strange places.