Stwat, my 1994 C180 has this same problem and it is a lower powered engine. The muffler collapsed 2 years ago and I had 90% new OEMs fitted except for the Cat which is Aussie cos the German one was too expensive. It makes a rumbling sound, and I am disappointed. When going uphill, no power and the kickdown is needed. It then makes more sound like a speedboat. On idle, the exhaust is not as quiet as Japanese or BMW cars. My other 1997 C180 is lots more quiet possibly with the improved engine and injection setup. Possibly the exhaust was also redesigned? This one only makes a bit of rumbling noise when in gear and at idle, foot on the brake.