I live in the philippines, and I bought this truck because there are many rough roads here.
One thing I noticed is that when I put a new belt on, it usually lasts about 150miles before the noise returns.
Also, one time, i put a brand new belt on, but went through a huge puddle of mud. Then, the noise came on almost immediately after wards (like 10miles after the mud).
Could it be that there are dust and fine sand type that collect on the grooves of the pulleys and the belt, making them squeak? Partinularly since the engine gives off some grime due to gas burn. So this will stick to the pulleys and the belt, and this grime in turn collects fine particles.
have you by chance cleaned the pulleys and belt teeth carefully with water/soap/alcohol to get them SUPER CLEAN and test for noise afterwards?