To diagnose the noises coming from a car, use a resource such as Dummies.com or PopularMechanics.com. Both sources have information on what certain sounds mean when an owner hears them from the car.
One example of such a noise is a high-pitched squeal. If this noise stops when shutting off the engine, it is likely due to a belt. Replace or readjust the belts, and if they have cracks, frays or the bottom is shiny, replace them. If the high-pitched noise continues after the engine turns off, it might be the radiator. In this case, check the rubber gasket under the pressure cap and replace it if necessary.
If there is a rhythmic ticking when the engine idles, turn off the engine, wait about 10 minutes, and then check the level of the oil. If there is enough oil, a mechanic may have to check the adjustment of the valves. If there is a knocking or tapping sound, this may indicate a loose rocker arm, loose bearing or piston with damage. If this is the case, it can destroy the engine. A soft knocking sound may indicate the wrong fuel is in the vehicle.
If there is a whistling noise coming from the car, look at the hoses for leaks, and if the whistling noise comes from the interior, there may be a weatherstripping leak.