I think it's chatter from the pad catching on the disc. On a downslope with the brakes lightly applied, it really started to make noise. It took a pry bar to get the caliper off the disc. It wasn't to the point of needing a sledge. BTDT. I took the caliper off, compressed the piston and bleed the system and it still happens but not as bad. I'm going to replace the caliper next week. Might have to consider new pads as well while I'm in there.
with the wheel on and that wheel lifted off the ground - if you apply brakes, then walk over and try to rotate the tire, if it spins freely then the piston is not seized. If it takes lots of effort to rotate the tire/wheel then it's stuck. do you have a little ridge on the outside of the brake disk (due to wear by the brake pads)? if you do then that's why it took the prybar to take the caliper off.
to me it sounds like you need brake pads, they are deforming in time and you get play between the metal support and the caliper mount where the pads are fitting in, just check to make sure you're not buying the caliper for the wrong reason.
EDIT: thinking about it and i think i might have got it wrong, the caliper would slide off and the pads would remain on the support, nevermind the wear on the disk - i was thinking of a different type caliper and pads (that have holes in so they slide out with the caliper, not the case with the Jeep ones, sorry for that). You're probably on the right track, i would change the pads as well if there is play between them and the mount though (not fitting snuggly in there).
