If it’s the first torsional and lateral bending mode of the fuselage you are referring to there is normally a lot of work done here to prevent this occurring,
With the Navy Lynx that I was involved with a few years ago , the tail wag was due to culmulative mechanical hysteresis in the yaw control run and yaw servo jack mount security (lack of it), it would kick in quite literally above about 100kts, we had a handful of “tank slappers” to use motorcycle parlance in the fleet before an oversight on an inspection periodicity on the yaw control run was unearthed. It could minimised on those Lynx by knocking off the No 1 Hydraulic system which provided the Yaw control run power.
I am not sure which aircraft you are referring to unless your name holds the clue but I understand a lot of mods have been done to play around with the torsional stiffness of the tailboom below the 1P frequency and nifty work with elastomeric mounts on the vertical tail to tailboom joint.