96 4.0 engine screech

I have a 96 Explorer with a 4.0 and 96,000. My problem is tracking done a hideous engine screech that has no consistant pattern. I have cahnged the belt, tensioner, and idler pulley but on various occassions the noise returns. I have sounded the alternator, water pump, A/C, and steering pump but I cannot locate the source. I can go a week without hearing it and then it will re-emerge. The only pattern is that if it is going to make the noise it will start on the start up. It would usually go away after higher RPMs but the most recent incident lasted through a 20 mile drive at 45 mph before it stopped. When trying to track it down I have even felt like it was coming from the backside of the engine but I cannot confirm anything. I could use some suggestions before I turn it over to the dealer and let them spend a week trying to figure it out and charging me out the tail pipe.