Serpentine belts are very susceptible to noise caused from any type of contamination including belt dressings sold in your local parts store. Even water/mud contamination from four wheel drives & from front wheel drive cars that have opposed mounted engines are common problems I see regularly. My best advise is to purchase a new belt & a couple cans of spray brake cleaner. Start by removing the old belt/belts being sure to note their routing pattern. Then THOROUGHLY clean each pulley with the spray brake cleaner. You should be able to rotate all of the pulleys except for the crankshaft to clean each one all of the way around. Use clean old t-shirts to wipe off any excess goo or whatever. Repeat the process as many times as you think you need to, to make then clean. Then install the new belts or belts & give it a try. Also note that serpentine belts need to be adjusted tighter than v-type belts. So if the truck has an automatic tensioner you will be ok providing that the belt is the correct length in the first place & since you said the tensioner was newer. If it has a manual tensioner you will need to make sure you get it good & tight so that your deflection is within probably about 1/2" using 30-40 pounds of force on a 10 inch run of belt. Give this a try & please let me know how the end result turns out!