My 2001 SLK320 has a chattering noise that appears: 1. When the engine is fully warmed up. 2. When the car is accelerating. 3. When the engine is between 2500 and 3000 rpm. The noise sounds like it coming from a vibrating piece of metal or plastic. It almost sounds like a duck quacking.(No kidding). I cannot isolate the location of this noise. It may be coming from the dashboard, cabin, or even the trunk. It occurrs with the top up or down. Any suggestions?