OK, bit the bullet and stripped the tumble down, ours is about 10 years old and is a CL432/1A so don't know if that's the same as yours Jen. We had nothing too loose!
There is a three pronged bearing mount at the back of the drum that is riveted on, each of the rivets has come loose allowing the drum to shift against the mount making a thumpy bang noise in one direction only.
I have removed the drum, ground off the rivets and propose to bolt the three legged holder back on to the drum but will replace the bearing first whilst it's in bits with this http://www.espares.co.uk/part/tumble-dryers/white-knight-(crosslee)/cl432-031243215001/p/1088/129/0/631740/100217/rear-drum-bearing-repair-.html
This fits inside the holder and is also slightly worn.
Hope it helps.
Kevin - Congleton, January 2009