As others have said check the pulley alignment and make sure the Alt pulley is installed correctly, using the correct spacer followed by the pulley, washer and nut. The nuts can be hard to lighten. I usually give it a quick zap with a 1/2 impact. Pulley alignment should be easy to check just by sighting across the pulleys from either the passenger or drivers side fenders. If the alignment is good, belt tension is correct and the alt pulley is not slipping I have had good success eliminating belt noise using the following method. Applying Comet or similar cleanser to remove glazing from the pulleys and or belts. Just pour a small amount of Comet between the belt(s) and pulley(s) (Easiest done at the crank pulley), then start the car. This may have to be repeated several times. Keep in mind this will make somewhat or a mess though its easy cleanup with water. Also do not get any in the alternator and make sure you have the air cleaner installed before hand.
Other thoughts: From my experience most belts available today are crap. They stretch under load like an SOB. The best brands IMO listed in order of quality are Bando, Mitsuboshi and Dayco. Seems most well known brands like Gates, AC Delco and Motorcraft, Napa etc. have started using who knows what actual manufactures. Some PN's are fine while others are freaking rubber bands. The problematic ones, no matter how tight they are tend to temporally stretch when engine speed is quickly increased such as on quick acceleration or change in accessory loading etc.
My 2 cents
Bill
