2004 BMW 745 Squeeking belt noise

Use a water bottle to dribble water onto one belt at a time when the squealing is occurring. The noise will either get worse or go away momentarily. When you identify which belt is responsible, besides the obvious looseness, look for any misaligned pullies by sighting down the belt between any three pullies. If the belt is not tracking directly in-line with the other sections, look for (typically) an idler pulley that is turned a little, usually due to worn bearings, allowing the belt to be misaligned. That causes the belt to slide sideways across the pulley as it goes around it.

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Possibly related, but the position of the engine isn't the culprit. It would be related if something got bent or reinstalled crooked. In particular, idler pullies on spring-loaded tensioner arms often twist when their pivots wear out. It is even possible for rust to develop in the pivot. The problem is already there, but the symptom doesn't show up until it is disturbed, such as when changing the belt.

This is kind of hard to explain, but if you sight straight down on the belt where it runs between two pullies, you should not see the same belt peeking out where it runs down lower between other pullies. Service manuals show using a straightedge, such as a yard stick, placed in front of various pullies. The straightedge must touch the belt every place it passes by. If there is a gap, typically of 1/8" or more, the cause should be diagnosed and corrected. That is enough to cause a squeal.

Do not use any type of belt dressing on flat serpentine belts. Once in a while it will quiet them down, but it is never a long-lasting solution. Serpentine belts are also prone to making noise when they get dust or small dirt particles embedded in them, particularly on the smooth backside. Belt dressing makes those particles stick better leading to even more noise in the future.

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