Im starting with this post because valuable information has been buried in our data base.
Meanwhile, industry and members re-invent the wheel trying to solve the same problem_ vibration at about 2000 rpm in the FD manual diesel.
Many members have reported their experiences and theories, but a remedy remains elusive.
As newcomers have arrived, I have asked them to inform themselves by using our search engine. This can also assist their repairer and save time and money.
Below are four links; 3 are past conversations, 1 is current.
The last link contains comparisons of repairs. No definitive remedy was ever found.
I note that the subject of mechanical resonance ( my favoured explanation) was first raised in 2015.
2010 i30 Diesel Transmission Vibration Replacement Clutch due to vibration noise (Was:Replacement Flywheel - 1.6CRDi) i30 2011 harsh vibration at 2000rpm i30 CRDi Engine Vibration