OK...so I've narrowed the symptoms down some more on my 98 CLK 320. When I'm at idle in park or neutral my RPM is about 700-800. If I put it in gear (R,D,3,2,1) it drops close to 500-600 and I get a slight vibration along with this humming noise. If I turn the A/C on it gets a lot worse. Any ideas? I have about 90k. I recently relocated to So. Cal so I don't have my indy back in Miami anymore....any help would be appreciated.