PeachParts Mercedes-Benz Forum - View Single Post

cantabridgian · #1 02-20-2008, 03:19 PM

My 1985 300CD (about 188k miles) has developed a nasty sound while driving on the highway, but I can never re-create it for my mechanic! Help? It occurs after driving between 50-75 miles on the interstate (usually I drive about 75-80mph, traffic permitting), never sooner. The sound begins as a soft thudding noise, eventually escalating into a giant banging, clanging sound. Interestingly, the noise only occurs while pushing the accelerator. When I take my foot off the gas completely - complete silence! - just the Benz cruising beautifully down the highway with no banging noise whatsoever. When I push the gas pedal again - bang! - there it goes. The banging will last until I pull off the highway and bring the car to a complete stop for about 15-30 seconds or more (like at a traffic light), and then it will go away completely until I get back on the highway. Then the banging will start again after about 30-40 miles driven or so, as before. The mechanic has assured me that it is not the exhaust. The right (passenger) side half-shaft was replaced by a previous mechanic about 5,000 miles ago. My current mechanic insists that this wasn't even necessary. Needless to say he (a) has never driven it far enough to hear the noise, and (b) says everything else is in good shape. Rear bearings have been replaced about 8,000 miles ago. Front end it in excellent condition. Meanwhile, the banging-clanging persists. Friday it started at mile 55 on a drive from MA to CT for work. After leaving CT, it started again after about 60 miles. Tuesday, driving from VT to MA, it started after about 93 miles driven (the first 35 miles on Route 4 at lower speeds, then about 58 miles at interstate speeds on I-89). By the time I got near Boston it was violent and horrible! And then after I got off the highway and stopped at a light...quiet again. Help!