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I replaced the belt on my 98 3.0 a few months ago due to the old one squeeling. The noise has came back. My dad told me not to use belt dressing and it is only temporary anyways. This sucks. I sound like a jackass driving around with that noise. It only does it under 1500 RPM though.
Is the belt tensioner doing it's job or is the spring in it weak? Also, have you checked for proper belt alignment? I assume it's the correct belt and there is no chance whatsoever that it might be the wrong length. Also, is there a componenet failing, such as a bad bearing, and the belt is slipping/sliding over it? Just my thoughts.
I had the same problem with a Buick. I must have used a pint of belt dressing. It didn't hurt anything but it is just a very temporary fix. A little extra tension did the trick.