with the belt removed the drum spins no problems, nice and smooth. Pulley is not loose. No loose bolts that I can see. The legs underneath and the springs on top seem seated well and nothing looks out of place.
I gave it a 1200 speed spin with nothing in it, and aside for some small vibrations at some point on the cycle it was fine.
Loaded it up with one towel and it rocked so alarmingly that the drum started knocking onto the metalwork at the front causing the noise.
So I'm confused. It didn't used to do this, so I assume something has changed, but nothing I can see.