Put a a goodyear gatorback and noise gone.
Let me also add that I had the same issue on my SL500 with 48k miles. I replaced the Conti belt twice and squeeked and chirped really bad. Replaced 10k miles ago with Goodyear Gatorback, No noise at all.
Now I have Gatorbacks on all three Benzes. The Conti belts really suck.