I know this thread is really old, but I'm having similar issues.
1998 Explorer XLT 5.0 (AWD) ~330,000mi
I've owned it the whole time and perform all maintenance on it.
I have two issues that may or may not be related. I thought the sounds were related and started changing things out one-by-one, but nothing is working.
First, a loud popping noise under the driver seat area when I turn left at slow speed (as in a u-turn). Even though it feels like it is under the frond driver's side seat, I suppose it could be anywhere in the front end suspension. It sounds "like" a gear tooth being skipped - very loud and distinct sound. It is NOT a repetitive chattering or clicking like bearings. The popping sound may occur several times during a u-turn.
Second, I've got a grinding noise in the front end like a bad bearing. The sound is mainly heard when under torque from accelerating, but when coasting sound goes away or is much quieter.
To attempt solving this, I've just replaced:
Both front CV axles.
Both front wheel hubs.
Front drive shaft.
Front differential.
Neither the popping nor the grinding sounds changed after replacing the parts listed.
Could grinding noise be the transfer case? The sound "seems" to be more in the front which is why I changed out all the front moving parts.
Could the popping noise be (1) one of the torsion bars, (2)Upper/lower ball joints, (3) Upper/lower control arms? I didn't notice any thing wrong with the ball joints or control arms when I changed out the CV axles, wheel hubs and front differential - each of which required same process over-and-over to remove and replace. Did I miss looking at something? How can I test each? I'm so frustrated, but convinced it's something fixable.