A common source of belt noise on the 4.0l is a separating crankshaft damper pulley. Check the spacing on the rear of the crank pulley to the timing cover. Normal space is about 1/16" to 1/8". Any more or less indicates a damper that is coming loose, the resulting misalignment causes noise. Sometimes the rubber is seen to be squeezing out between the inner hub and outer ring.

Hope this helps!