I replace the axle bushes on mine. I put in polyurethane Powerflex jobbies. This got rid of the knock, but the top mounts then started, it was probably always there, but was drowned out by the axle ones. To replace the axle bushes is a complete sh1t, but as long as you've got someone to help, then you'll be ok (I didn't, so I swore alot instead!!)
Lewy