The brief
The E200 was squealing on every cold start, a high-pitched chirp from the front of the engine, and once it was warm the noise would come back as a screech the moment the aircon clutch and the alternator both pulled load at once. Two clear signs that the belt drive at the front of the engine has aged out.
The accessory belt is the single ribbed belt that drives the alternator, the aircon compressor and the water pump off the crankshaft. A spring-loaded tensioner keeps it tight, and idler pulleys guide it. When the belt glazes and the tensioner spring weakens, the belt can't grip properly under load, so it slips, and slipping rubber is what you hear.
The diagnosis
With the belt off the engine, the rib face was glazed shiny and cracking across the ribs, well past the point of replacing. The tensioner tested weak, the spring no longer holding the belt at proper force, which is exactly why it slipped under load. The idler pulley spun smoother but it's on the same age clock as everything else.
So this was a whole-set job: the belt, the tensioner and the idler all together. Putting a new belt onto a tired tensioner just gives you the same noise again in a few months.
The work
Belt tension was released, the old belt and the failed tensioner came off, and new Mercedes-spec parts went on, including a new idler pulley. The new belt was routed correctly through the accessory drive, since a misrouted belt is its own kind of noise, and tensioned to spec.
Then the engine was run through a warm-up cycle with the aircon and other loads switched on to confirm the belt drive ran silent, cold and hot, under load.
The outcome
No squeal on cold start, no screech when the aircon kicks in, and a silent belt drive right across the rev range.
The E200 went home with the front of the engine quiet again. A belt drive done as a set, belt and tensioner and idler together, is a job that stays fixed, rather than chasing the same noise back here every few months as the next worn pulley gives up.