There are many causes that can force serpentine belts to become noisy. Sometimes this is due to pulley misalignment or even a wobbling accessory pulley. Also incorrect alignment of the drive belt can result in noise and premature wear.
Do not use belt dressing
Often these belt dressings will start to break down the rubber on the belt and replacement will be needed. These engine belt noises can be intermittent or constant noise. Often this occurs when the drive belt slips on an accessory Pulley under heavy load conditions.
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Engine belt dressing may increase the slipping but silence the noise because it is considered a lubricant. Use the above 5.4 Liter belt diagram to replace the serpentine belt instead of spraying belt dressings all over the old one.
5.4 Liter belt noise
Also in the case of an alternator or another accessory that has a worn sticking or even seized bearing can cause the belt to slip over the top of the Pulley instead of rotating the accessory properly.
Also fluid leakage onto the drive belt may cause the same condition where the belt will slip over the accessory instead of driving it properly. Most often you will see this with the water pump. Sometimes the 5.4 Liter engine will get a slow leak from the water pump and as the pulley rotates it sprays on to the belt.
When the accessory drive belt is too long this will allow the belt tensioner, number 7 in the 5.4 Liter belt diagram above, to be pulled all the way to the travel stop. This may cause an incorrect tension and allow for slipping condition that causes nasty belt noise.
The Ford engine belt tensioner has an indicator that shows a normal range of operation. If this indicator is out of this range this is a sure sign that either the belt has stretched excessively or the wrong part has been installed. You should also check that the tensioner has smooth operation and is not binding or sticking when you rotate it.
Additional engines are covered on my drive belt diagrams page.
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