So far I have replaced 2 belts not counting the many the dealer did over the years, but my son found that the two belt pulleys ,the alternator and the pulley right above it are misaligned! He calls it co-planer alignment. Where as the top pulley on my 2005 6 cyl is 1/4 inch out of alignment from the other.
We both were suprised at how far misaligned they were. So after replacing a serpentine belt ,it would be about 12,000 miles and start making a squeeling noise at start up and until it gets warm(the belt) or when it rained out it would do this for hours and then may stop. So if you have belt or tensioner sounding problems it might be the alignment of the pulleys
To see, put a straight edge on one pulley and it should contact the other pulley with out any gaps between the straight edge and pulley
The sad thing is... we do not know how to compensate the gap at this point,
but a new belt solved the noise for now
Any one come across this one and any remedies