I just replaced all my belts (3 total),
1. right after that whenever i turn on my ac it would squell.... so tigtened the AC belt just a bit.... squeelling sound was gone after that
2. then i found out that when ever i turned left or bust a U (to left) i would hear squeeling sound, so i tightened the POWER STEERING belt ( poof , the sound was gone)
it seems that the belt should be pretty tight, i mean not too tight,but when you push on it, you can feel the resistance.. i think you should be able to push it down about 1/4 of an INCH. but test it out.. if i were you i would tighten the belt very little at a time, and then see if the sound is still there.
with mine, it was obviously slipping on the pulleys.
(you can tighten the power sterring pump belt with the tensioner thats on it, but you have to loosen 1 or 2 other bolt just a tad bit) with the AC you have to tighten the bolt thats facing down.
good luck...(free tibet)
(also don't tighten the bolt that has the hole where the tensioner slides in, i over torquedd it and it broke, so had to buy that from nissan dealer for $12 bucks... just a small bolt...ohwelll.. live and learn , i guess