only thing I can think of is the axle... perhaps you have a pinhole in one of your boots, and it's getting hot after a long drive and expansion is causing it to click?

also possible is the ebrake shoes are damaged, and after heating up they contact something? or heat could be causing the rear bearings to start to click...

get a good floor jack and carry it around with you, drive until you hear the noise, then park and quickly jack up the wheel and spin the tire by hand and see if you can locate the noise. carry wheel chocks with you and possibly even a jack stand or at least throw the spare under the side of the car for some protection from the car falling off the jack...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!