The brief
The Golf had a high-pitched squeal when starting the engine and under acceleration, and a look at the belt showed it cracked, frayed and glazed shiny. He brought it in before the belt actually let go. The accessory belt drives the alternator, the aircon compressor and the power steering off the front of the engine, kept tight by a spring-loaded tensioner. As the belt ages it glazes and cracks, and as the tensioner's bearing wears it stops holding the belt steady, which is the squeal. Left long enough, a slipping or snapped belt can let the engine overheat, leave the steering heavy, kill the charging and the aircon. A belt that's glazed and squealing is a set on its way out.
The diagnosis
The belt was glazed and cracked along its rib face with fraying at the edges, classic age-and-heat fatigue, and the tensioner had a wobble on its bearing. Doing just the belt would leave a worn tensioner ready to be the next problem behind it, so it was a set job, belt and tensioner together, rather than back here in a few months.
The work
The tensioner was released, the old belt slipped off, and the tensioner swapped for a new genuine VW-spec unit. A fresh belt went on, routed correctly through every pulley, and re-tensioned to spec. A road test confirmed the squeal was gone and the belt drive ran silent across the rev range.
The outcome
No squeal on a cold start, no slip under load, and a silent belt drive across the rev range. The Golf went home with the front of the engine quiet again. The accessory belt and its tensioner wear together, and a snapped belt strands you with no charge, no steering assist and no aircon, so doing the set on the first squeal kept it to a tidy job.