I've just got rid of the vibrating noise that appeared the instant I drove out the mot station. Took the rear prop off in the end and it went, put it back on and it's still OK. New one now, sounds like a dry bearing whining at the back.
Id use all 4 fixings in the seat base . If you tried going backwards and forwards with just 2 fixings, it tends to crab along the runners putting strain on the nylon that holds the ball bearings in the runners.