Just a thought chaps, but my previous Merc got a bit noisy in the same area, so I got my garage to flush the engine on the next oil and filter change and then to be sure changed the oil and filter again after another 1000 miles. For me this helped an awful lot, but it was a much older (25yrs+) engine. Not sure if its available where you live, but engine flush can be purchased at most motor factors or "DIY" garage stores in the UK. (Wynns or similair)

Regards

Shane