There is a rattle from the rear of rear of my A-Class, what could it be and how can I stop this?

There are a number of things which can cause a rattle. Each are listed with the cure underneath.

Rear Seats
Cured by one of three methods, first, remove the rear seats and wrap a piece of felt around the fixing bar with either glue or tie wraps. Then refit the seats. (DavidH)

Head rests
Remove from the seats and pull the chromed steel posts further apart, so once refitted this provides a tighter fit. (MrLapou)

Spare Wheel
Remove the wheel and place a piece of folded cloth or foam block about 20mm in depth underneath the wheel. You might want to make a hole in the cloth or make the foam ring shaped, so the wheel can be screwed back into place. (MrLapou)

Rear hatch interior
Remove the rear trim and place pieces of foam at strategic points where the trim flexes the most. (MrLapou)

Rear Lamp Cluster
Remove rear lamp cluster to reveal the black holder with bulbs and metal tracks. Carefully bend the holder so it's a little banana shaped, just enough for it to seat tightly when it's clipped back into the body work. (MrLapou)

Boot Locking Mechanism
Loosen the two star drive screws and moved the locking plate in about 3/16" making sure the boot still fully closes and locks. (Lofty)