Clunking when gears change

Hi Guys,



I own a E46 1998 318i It's one of the first batch built I think. it has a normal auto box (assuming its the same as the E36)



Anyway, when I drive and the car clicks down a gear It makes a nice thud\clunk\thud coming from the back of the car.



I believe it does this onn every gear down change.

Also When I am sitting stationary and changing from park to drive or reverse It also makes this clunk.



Any Ideas?

Thanks