Since you have a warranty that will run out soon: If the vehicle doesn't make the noise when it's on a hoist in the shop, but it makes the noise when you're driving it, the obvious answer is -- hve the mechanic go on a test drive with you, and point out the noise to him.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.