Yet another vibration @ idle

[lp01]

Dear Xoutposters,

I've done some considerable research on this issue but there seems to be no unique, objective causes to this issue. My car is a 2001, FA71 with a 3.0 diesel engine.

Here's what I found concerning this particular vibration:

-It happens only when stopped, foot on brake and gear "D" ~(600 rpm); the vibration is sometimes audible;

-In "P", "N" or "R" it does not happen, although in "R" there is a slight vibration not present in "N" or "P"; nevertheless this vibration is not audible. Whether in "P", "N" or "R", the engine idle speed is ~660 rpm

-It does not occur when car is in a positive inclination slope (only flat or negative inclination)

-There are days when it occurs and others it doesn't; not related with outside temperature though, as far as I have noticed.

-Vibration is not sensible to A/C settings, meaning I can turn it on or off with no impact on it; besides, the A/C does not influence the idle rpm at all. The engine control seems to "compensate" for the A/C immediately.

So... I have 2 questions:

Question 1. Since the engine is idling at 600 rpm in "D", as opposed to 660 rpm in all other gears, do you think this could be the cause?

Question 2. Independently of the low idling rpm being the cause or not, is it possible to raise ~50 rpm when the transmission is engaged in "D" without affecting the rpm in all other gears?

Thanks in advance for your input . I have scheduled a visit to a dealer within one week, but I would like very much to have your insights on this issue.

Cheers