While there have been a few novel ideas suggested so far that I will be sure to remember in the future if the usual suspects don't pan out. As has already been said this is most likely a collective minimum friction problem causing collective bounce. The load bounces on the line inducing more of a vertical into the airframe than the aircraft on its own would normally see with just an internal load. Pylon rock is more of a cyclical lateral vibration not a vertical one. A bad 5th mount usually is the cause of pylon rock.
Let me re-iterate: check and adjust the collective minimum friction clamp before further slinging ops, aircraft have been lost because of this! While you're at it (i.e. your engineer) remove and clean up the friction clamps with some scotch-brite, it does wonders for collective feel-no more notchiness for the next several hundred hours hopefully.
Last edited by cbox chip; 11th Nov 2012 at 04:55.