Had something very similar in a 212 awhile ago. I was an engineer working on a seismic job, we had a 212 going into what felt like an airborne ground resonance when coming into a hover with a heavy long line load on, not a nice feeling. Only happens with a load on.
Tried everything, new mounts, rigging etc. Replaced the transmission and the problem was fixed. The transmission had been assembled with mastinox between the splitlines . With a load the transmission was being pulled apart, the matinox was then a lubricant and the cases started moving.
Have the engineers check torque the splitline bolts, the washer stackup is an alloy then a steel, the alloy tends to get squashed and you loose torque on the bolts. If you have lost the torque retouquing may help but the splitlines may need cleaning to totally fix the problem.
Hope this helps