just my 2 1/2 p.
what Brian Schlorff sounds more reasonable than my idea which is on the
line of Kelly Alford, if the mounting bolts are fine you could have a bad
or sticky shock. as you come to a stop the car leans forward and after it
stops and as with Brians mounting bolt solution, gravity brings the rear
end down the shock or its mounting point would go clunk. try bouncing the
the rear up and down as you would with a regular shock test.
its not likely the problem because the clunck would be happening more often
but its worth a check.