you may have to have the Jeep running but if you can hear it with a slight pressure, you should be able to leave it in park and walk to each wheel and have someone press on the brakes and hopefully you'll be able to hear where it's coming from. then i'd check the caliper - if they left a bolt out it'll cause it to shift - after that i'd check the brake pads and make sure the pistons are activating all the way, if the piston is not activating all the way the pad will be loose and slap the disc surface.