Lu mentioned that elastomeric dampers must be fairly "matched". Fairly shortly after I got my private helicopter rating in 2001, I was spooling up the Schweizer 300C that I was flying at the time. At some point during rotor spool-up the ship started shaking something fierce, and to be honest I wasn't quite sure what to do. I believe I pulled the mixture control to shut the engine down, disengaged the rotor, waited for everything to stop, and then went through the startup again and experienced no problems.
Shortly thereafter the owner of the ship elected to replace the main rotor dampers (perhaps some other renters had similar problems? I never heard why he decided on this action), which was roughly $12,000. That was my first lesson in unscheduled helicopter maintenance costs 8^) . I have to say that it made me a little less inclined to buy a 300 model as a personal ship, but I still love to fly 'em.
Dave Blevins