The brief
Mr Goh's C180 had a screeching squeal every morning on a cold start, and even after a new fan belt the noise was still there. He brought it in. When a fresh belt doesn't fix the squeal, the belt wasn't the culprit, it's the pulleys the belt runs over. The drive belt runs the alternator, the power steering and the air conditioning compressor off the crankshaft, over a tensioner pulley and one or more idler pulleys. Those pulleys have bearings, and over the miles the bearings get rough and dry, so they squeal, especially when everything's cold and stiff on start-up. A new belt slipping over a rough pulley still squeals, so the tensioner and the idlers, the worn pulleys, need replacing.
The diagnosis
A check confirmed it, the belt tensioner had lost some of its spring with a rough pulley bearing, and the idler pulley bearing was rough too, which is exactly what makes a cold-start squeal that a new belt won't fix. The belt itself, the recently fitted one, was fine. That's a tensioner and idler pulley replacement, the worn pulleys renewed, and the belt routing checked.
The work
The drive belt was released, the old belt tensioner and idler pulleys removed, and a new genuine Mercedes-spec tensioner and idlers fitted, the belt seated properly across every pulley and the routing checked against the diagram. With it all back together the engine was run to confirm the belt tracked true and ran quiet. A road test, including a cold start, confirmed no squeal, no chirp, and everything the belt drives working as it should.
The outcome
No more cold-start squeal, the belt running quiet and true over fresh pulleys, and the accessories all driven cleanly. The C180 went home with the noise sorted. A rough pulley bearing squeals no matter how new the belt is, so changing the tensioner and the idlers put it right, the part that actually needed doing.