I have an 89 300se with the same problem. Only started 200 miles ago, and seems to be getting worse. My noise begins around 35 mph and increases with more speed. Noise continues even when coasting in neutral and goes away around 25 mph. I had the wheels checked for balance. Tires checked for bad belts, and bearings are new. After many searches, the drive shaft area keeps coming up, and that's next. After looking at the manual, it doesn't look like a diy project. I'll be anxious to see any more helpful posts.