You mention that the vibration comes on at about 4000 in 3rd --- whats it like at 4000 in all of the other gears, reason for asking is that if you are at 4000 in 3rd and then change up, the RPM will drop significantly due to the ratios in the box (thats what a gearbox does) does the vibration come back at 4000 in 4th or does it go away all together, never to return until you get back into 3rd again.