I have done some more "investigation" into this problem and also called the dealer. I determined that the noise is definitely engine load related. So that suggests pinging. On the other hand there are 2 reasons that I question pinging:

It starts even at very moderate load levels (slight hill) and also, at its worst doesn't really sound like any pinging I have ever heard. It seemed worse around 2500 to 3000 rpm, so I tried a test where I coasted at around 30mph and shifted into neutral and reved up to that range, nothing, no noise. So all I know is the tranny has to be in gear. Some times it almost sounds like a bad bearing. Very strange.

I called the dealer to see about making an appointment and talked to a senior tech there. After explaining all this and telling him I had been using 92 octane as the manuals says to do (actually says 91) he immediately said to try "high quality 87 octane". I had been using Arco 92 octane which has 10% ethanol (around here anyway). He claims that the name brand gas doesn't have the ethanol which surprised me since I though it was mandated by law. Anyway, he says this 87 octane idea for the 4 cylinder cars comes from high levels within Mercedes and is not just his idea. When I questioned this wisdom since it is a high compression engine he said it is not "really very high compression". Unfortunately I have a full tank of the 92 and don't drive huge miles so it will take me awhile to try the 87 octane. Does anyone have any opinion of all of this???

Thanks,

Mike

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra