this is how we found the bearing noise on my 300SD. take a golf club or other solid metal rod. Place the handel end of the golf club(making sure no rubber grip is between the rod and the engine) and put your ear up to the other end. put the rod in differant places. You should be able to hear the engine running, and also hear any bearing noises. You can't really put it right on a pully but you can rule out other stuff. turned out to be the alternator on mine. you could here the bearing through the club even though it wasn't making noticible noise otherwise.
R