rumble vibration in 2nd gear

hi my 56 plate 1.9tdi Octavia 34k, occasionaly I get a rumble or slight vibration from engine or cluth when acccelerating in second gear. Been back to dealer and drove around the block 5 times and it did it once. admittedly its not bad but never had it on my previous octavias. Dealer said to give it more revs in first, and explained abiut the clutch plate and flywheel. Still there, could this be the beginnig of the dreaded mass flywheel problems I read about, and should I be worried