Been a long time but some things to possibly check include loose skid saddle mounts - these can set up a 2 per rev bounce on a tarmac...lift to hover or jack and get a maintainer to give the skids a good bang and shake to check security of mounts and a good check.
As Evil Twin states, similar to the B412, holding about 1/2inch or so of collective can also markedly reduce this if saddle mounts are a problem as well.
Another possible cause of 2 per rev in huey/teetering head systems with transmission mounts, can possibly be worn pylon damper and/or transmission mounts. There is a hover check for the pylon damper but unfortunately I don't have the old test flight schedule handy with me at the moment to check. Something along the lines of a smart 2inch cyclic displacement and the pylon rock should stop within 2-3 oscillations of the Tx (but don't quote me on that).
See if any of the above may be a culprit.
cheers