It only happens in 3rd and 4th gear and goes slightly faster or slower depending on the speed i am going. It's more noticable when i am travelling along steadily rather than when i am speeding up or slowing down. At higher or lower speeds it doesn't happen at all.
Worn drive shaft joint - typically these won't produce any vibration unless badly damaged but they can be noisy or 'clonk' for a long time beforehand, turn the steering to full lock and look at the drive-shaft rubber boot there should be no rips or tears in it and the suspension should be clean and dry of any grease. If you reverse at a reasonable speed with the steering on full lock a worn joint will 'click'.