I assumed the sound you described was coming from the engine. Can you tell if it's coming from the area of the engine or the muffler? I would think it would be easy to tell if you can hear it at idle. Hopefully he's right about the muffler baffle as that would be easy to fix.

If you are hearing strange sounds from the engine, I would recommend finding a good independent MB shop in your area and have them check it out. I can't guess what would need fixing until the problem is identified.