The easy way to determine whether the noise is from a belt or from a pulley is to shoot the contact surface of the belt(s) with a little wd-40, being careful not to get it all over the pulleys. If the noise goes away, tighten or replace your belt(s). If the noise doesn't change then you need to look at the bearings in your idler pulley(s).

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes