Squealing NOise

I have a relaively high pitched "squealing" noise coming from the engine on my '03 525i. It only makes the noise when the car is idling in drive or reverse. Once I accelerate, it goes away. When I stop at a light, it returns. It rarely makes noise in park, but sometimes. Any thoughts on what could be causing? I suspect the serpentine belt, but not sure. Any help wuld be greatly appreciated.

Try some belt dressing on the serpentine see if that helps, or if you want to really break the bank...replace it. :D

If you want to know for sure, take the belt off. Not a good idea to do it for very long though. In fact, I wouldn't recommend it.

All kidding aside that's my guess too, it's probably a little slack. Without the engine running push down on the belt, if it goes down more than about an inch (2.54 cm for those fellow metric users, or about a finger width), it's worn and should be replaced before it gets worse.

Your car has an automatic tensioner so unless there is something wrong with that or the belt you should always have the correct tension on the belt. My guess is there is either something wrong with the tensioner, the belt, or I think your car has two idler pulleys that can go bad and squeek. One is on the tensioner arm and one right by it. If belt dressing does fix the squeal, then check the condition of the belt, if that is good then you need to see why the tensioner is not keeping enough tension on the belt. How old is the belt maybe you ought to change it.

Replacing the belt on this car is relatively easy though harder than most vehicals, and you don't have to take the engine fan off like most say. I did on mine in a little over an hour with only a few cuss words involved. Hardest part of the job was getting the right wrench for the tensioner puller to pull it in so I could slip the new belt on. On mine it was an 8mm hex key. The pulley on mine is hydrolically dampened so have to pull it in very slow with significant pressure. Don't expect it to move immediately like the air conditioner tensioner puller.

If it turns out you have a bad pulley then you will probably have to remove the radiator, and engine fan to get enough room to change them.

JollyrogerX is correct except when changing the belts, I would mess with keeping the fan on the vehicle. Take a 32mm combo wrench and slap it with your hammer (remember, reverse threads). This will knock the fan clutch loose. Then spin the fan off. Removed the plastic rivets on both sides of the fan shroud and lift up on shroud (there will be coolant hoses connected so you cannot removed completely without disconnecting cooling system (big mess... too much time). Remove fan, take a 6" or slightly longer extension and brace up the shroud. Now you have easy access to the belts without such a tight space to deal with.

Removed belt and spin all pulleys, listening for noise. Sometimes its hard to hear. Depending on how many mile and how much you care to spend, you might as well replace the pulleys.