BMW 325xi Loud squealing or squeaking is coming from timing belt Inspection at your home or office.

How this system works:

The timing belt is a toothed belt that keeps your engine synchronized (in time). It ensures that the camshaft and crankshaft are in synch during operation, and that the valves open and close at the right times in relation to piston movement. As such, it is perhaps the single most important maintenance item on your car and should be replaced at the manufacturer’s recommended service interval.

To sum this up in a nutshell, the piston and valves in your engine essentially take up the same space. Obviously, they can’t do this at the same time, or they’d crash into each other and cause serious damage. The timing belt ensures that their movement is timed correctly. When the belt breaks, timing is thrown off. The most common outcome here is bent valves (due to impact with the piston), but it can also cause damage to the cylinder head, or even to the camshaft itself.

Note that some automakers have switched to using timing chains rather than timing belts. Metal chains offer much better lifespans, and should last for the life of the engine in most cases. It should also be noted that timing belts do not squeak. The belt is toothed, so it doesn’t slip like a serpentine or V-belt. Any squeaking noise from around the timing belt cover must come from something else.