Pull the rear wheels off and check the brakes. Something may have broken or fell apart.
YOU WILL FEEL A CLUNK WHEN THE VEHICLE IS PLACED IIN REVERSE OR DRIVE AND IS ACCELERATED WITH THE BRAKES DEPRESSED
This happens on my 2002 mk5 fiesta. The front brake pads move very slightly in the caliper, so when you reverse and apply the brakes you'll hear a slight clunk. I had this repaired by ford as my car was under warranty - replaced pads and noise went away.
bad synco rings
sounds like a motor or transmission mount is your likely problem
Worn trans or diff parts. If it gets worse, have it looked at.
it could possible be the U-Joint which is a cheap and easy fix. it's connected to your drive shaft.
when you change directions from forward to reverse it makes a clunk sound and there is a slit delay in movement.
If the ticking is in time to wheel speed and most noticeable on turning I would suspect an outer CV joint. An inner axle joint might clunk when shifting into reverse. Maybe they're on the same axle. Other possible causes of clunking are worn-out motor mounts.
it may be the rear tranny mount
You're a clunk-head...
Check the wishbone bushes, check the hub bolt to see if the drum brakes are secure. or scrap it.
Jammed brakes...get them opened up and cleaned if the cars ben idle for a while...or could be the CV joints,try regreasing them