Horrific squealing when car is first turned on

The horrific squealing sound is your belt rubbing on on some pulley. Typically this means that either

1. The belt is bad.
2. The belt is loose.
3. Some pulley has seized.

If the belt is cracked or feels like its dried out then its probably time for a new belt. There's also a product called belt conditioner that you can spray on the belt. If this fixes the squealing then its likely just a belt problem and you should replace it. They're typically pretty inexpensive so if in doubt it won't hurt to replace it.

If the belt is loose, then tighten it and see if the noise stops. If so and the belt seems to be in good condition then you're probably ok.

If the belt is not loose, then whether or not the belt seems like it needs to be replaced, the next step is to take off the belt and check all of the pulleys. The most likely culprit would be an idler or tensioner pulley.

I am a former accessory drive engineer at Gates. The squeal you hear can be due to a couple of reasons.

1. Your belt has worn out. Depending on their construction, (usually either cotton, polyester cord or aramid cord), belts stretch 2-5% of their length and wear down deeper into the grooves. This causes their friction to change and results in noise. Check your tensioner position. They usually have small indicators which will tell if the belt has stretched past its useful life.

2. You have a pulley slightly misaligned. This can cause that chirping sound which is the belt trying to climb a tiny amount out of its seat on a pulley. There may be a bearing which has worn out that causes this condition, or the vehicle may have just been built at the edge of acceptable tolerances.

3. Your tensioner is worn out. Change your belt tensioner when you change your belt. They have springs and friction dampers inside that adjust the belt tension to that which is necessary for your belt dynamics. They also smooth out the rough belt jerkiness that is caused by the engine rotation, which varies by 200 RPM between a few hundred microseconds.

Please do not use belt dressing. It is not recommended by belt manufacturers as a cure.

Try a belt which has a 'fuzzier' texture on its surface. These are more forgiving of misalignment and better in wet conditions. This will affect the friction level and thus eliminate the noise.