suspect clutch disc
clicking from rear end on 1997 Ford Expedition
Did you ever find out what was causing this problem?
Check your rear AC unit.
if your vehicle is equipped with rear a/c there might be a problem with the door actuators on the rear a/c there are two to find out witch one change the a/c from cold to hot to see if the sound goes away and do the same with the fan speed. they are easy to replace and cost around $50.