could this perhaps be described as a "ticking" sound? if so its probably the 4.0's infamous pushrod rattle (or lifter tick if you like).
it would help if you could pinpoint the noise. with the engine running (stay clear of fans, belts, pulleys, blah blah), use a long screw driver with the handle held to the ear...place the pointed end on valve covers, belt tensioner pulley, each injector, the engine block, the oil pan....ect.