Hi JVY, looking up at the gearbox from under the car just behind to tail shaft you can see the selector rods which link to the main gear selector rod via a bush. If you can get your hand on rod from gear lever & rock it back & forwards, there will be some slack (about 4-mm if like mine). This is excessive,should be 1-2mm max & when travelling over 45mph the natural movement of the box & rod causes chatter. I could not see clearly whether it is a nylon bush or phosphor bronze but which ever one it is i am sure this is causing the problem.If i find one i will let you know & likewise if i make one. I think the prop will have to come down to get access but all external box work so not too involved. Cheers Mark