Why does my car squeal when I start it on a cold morning?
A sharp squeal for a few seconds after a cold start, fading as the engine warms, is almost always the auxiliary drive belt slipping on its pulleys. It is one of the few car noises that is both very recognisable and usually cheap to fix.
The auxiliary belt — often called the serpentine belt — is a single ribbed belt driven by the crankshaft that turns the alternator, water pump, power steering pump and air conditioning compressor. It transmits power through friction against the pulley surfaces, so it depends entirely on correct tension and a clean, undamaged contact face.
Cold makes slipping more likely for several reasons at once. Rubber is stiffer and grips less when cold. Any moisture or frost on the pulleys acts as a lubricant. And the alternator is under its heaviest load immediately after starting, recharging what the starter motor just drew. Once the belt warms and the electrical load falls, the squeal stops — which is exactly the pattern people describe.
The usual causes are a belt that has aged and glazed to a shiny, hardened surface, a worn automatic tensioner that no longer applies the right force, or contamination from an oil or coolant leak onto the belt.
It is worth acting on rather than living with. That belt usually drives the water pump, and a belt that snaps takes the coolant circulation and the alternator with it — leaving you without charging and overheating within minutes.
Belt dressing sprays quieten the noise temporarily and are not a repair; they can make glazing worse.
A squeal that appears only when steering points at the power steering pump instead.