I suspect my 03 Explorer 4.6 may have the same thing starting to happen at 100k. It's made noise since I bought it at 85k. My noise sounds like a piece of broken catalytic converter rattling around in the cat, or maybe like a torque converter problem of some sort. It's load-sensative, and seems to mostly do it while accelerating from 0 to 60. The noise always lets off as the tranny shifts (I'm thinking due to lack of load), and goes away as the truck enters 5th gear, especially with the converter locked up.
An ex Ford tech told me he recognized the noise, it was a common problem, and was the timing chain guides. Once he told me that, the symptoms seemed to fit. There's no rod-knock type of noise. Just a SHHHHHHH noise when accelerating. It doesn't seem to be getting any worse, oil changes don't affect it, and it goes away above maybe 2500 rpm.
Can you change tensioners or guides without having to retime the motor? I'm hoping to not have to buy the 2v cam timing kit. How much work is it on an 03 explorer? I've done this job on 4v Mustangs, but I'm guessing the explorer chassis is a bit tougher to work on, due to it's height and size. Couldn't you just paint the chain links and the timing gear before you disassemble that tensioner, so you don't have to retime the motor from scratch?