My car has started making a faint knocking. It sounds mettalic between 1k-3k revs but then goes. Doesn't do it on idle or in 5th or 6th strangely. I've just had it checked out and they've said suspected layshaft and not dmf. Now, is this common? Its just turned 100k. Also, how can I find my gearbox code? Its a 2006 car.