The brief
The A 45 AMG had a loud squeaky noise from under the hood, the kind of chirp or screech that comes up on a cold start or when the engine's working hard, and on a performance car like this it's exactly the thing you want sorted before it lets go. He brought it in. The drive belt, the long serpentine belt at the front of the engine, runs the alternator, the power steering and the air conditioning compressor off the crankshaft, over a tensioner and idler pulleys. Over the miles the belt hardens, cracks and glazes so it slips and squeals, and the tensioner loses its spring and the idler bearings get rough. A squealing belt is a slipping belt that's near the end, and a belt that fails can wrap up and damage other parts, so a proper job does the belt plus the tensioner and the idlers as a set.
The diagnosis
A check confirmed it, the belt was glazed and cracked from its miles, slipping under load, which is the squeal, and the tensioner had lost some of its spring with a rough pulley bearing, the idler on its way too. So it was a full drive belt set: the belt, the tensioner and the idler pulleys together, not a belt alone that would soon be chirping on the old hardware.
The work
The old drive belt was removed, then the tensioner and idler pulleys, and a new genuine Mercedes-spec belt fitted along with a new tensioner and idlers, the routing checked against the diagram and the belt seated properly across every pulley. With it all back together the engine was run to confirm the belt tracked true and ran quiet. A road test confirmed no squeal, no chirp, and everything the belt drives working as it should.
The outcome
A fresh drive belt running quiet and true, a new tensioner holding it properly, no squeal, and the accessories all driven cleanly. The A 45 AMG went home with the belt sorted. A squealing belt is a slipping belt that's near the end, so changing the set put the running quiet and headed off the belt letting go and damaging other parts.
