The back [cylinder block] side of the flywheel is often left in an 'as cast' condition & the whole thing is factory balanced that way. If the back face of the flywheel is machined it can easily remove more material from one spot than another due to the previously unmachined cast surface being well & truly uneven.

This then would throw off the factory balance since the factory job previously corrected the imbalance caused by the uneven 'as cast' areas

My flywheel lightening was done on a lathe...and of course it should remove the weight evenly as you said....

So, dd, if flywheel's not the case, any idea what can be wrong?...( except for the engine balancing )

i always check my wheels if there is lots of shaking...did balancing on the wheels then it does wonder..

when did the last time u change ur mounting bush?

DV, it's not vibration due to wheels, that kinda vibration i know how to differentiate. This is due to the engine itself.

It happens only beyond 4k rpm on which ever gear you're on. The gearstick rattles and has a buzzy noise more like a vibrator / dildo.

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LOL did you get your lightened flywheel balanced?
[edit] read another earlier post doesnt matter if u took it off evenly it will still unbalanced mine was out a bit not much though...