I haven't resolved it yet. I spoke to the local dealer, and the service manager said that he knew exactly the sound I was describing, and he had heard it once before on a 98 or 99 e300. They were certain the whine had something to do with the lubrication system, and were prepared to replace the oil pump under warranty (the car was relatively new). The owner moved, so he never replaced the pump. He said the trouble could be the pressure relief valve, but that is also with the oil pump. I have searched various forums, and I have seen maybe half a dozen similar posts to ours (but no clear answers). I think there may be a difference between your noise and mine. Mine gets louder as the engine reaches operating temperature (presumably because the oil cooler thermostat opens). I saw a post about a technical service bulletin addressing early 98 models with an oil pump whine or howl that is quieter after warmup. That seems to fit yours well. The cure was an upgraded oil pump. Although my noise increases with temperature, I wouldn't be surprised if the cure is the same. I changed my oil after only 2,000 miles, and I looked at the filter and ran the oil through a fine mesh screen. I didn't see any metal, so I am not as worried as I was that I am causing engine damage, but I am not giving up on finding the source. I hope someone reading the forum will be able to help us. If not, I'll be giving the dealer a crack at it since he had some familiarity with the problem.