Some vibrations are tough to diagnose.
Had one once on a B747. Engineers kept trouble shooting the engines with fan balance etc. Still it persisted intemittently over several flights for weeks. Finally a wing flap fell off on approach. (fatigued bolts) Also noted fatigue on an engine tail cone attachment. Engine sent to shop and when they opened it up they found a turbine rotor tierod had fatigued and left the broken ends tumbling inside the rotor.