Ref Vertical bounce.
The OK phrase used to be vertical "bounce"
as opposed to lateral "shake".
One very insidious cause for vertical bounce that happened to me on a Bell 206 during a lifting mission was Hydraulic Failure. As an instructor we usually demonstrate hydraulic failure as being a black and white affair, in other words the Heli is in manual or has control assistance. BUT BUT BUT in view of the construction of the splined pump supplying the hydrabolic power there is a very interesting grey period between the black and white. This results in the most baffling flight characteristics as the hydraulics are engaged/disengaged as the spline gradually disintegrates.In my case I was convinced that the vertical bounce lateral shake combination was leading to airframe failure. Without hesitation I released the u/s load and landed. The engineer and I went over the systems with a fine tooth comb, Nothing ,apparently, wrong.We then restarted the hel and realised that the hydraulic pump had failed properly and all was revealed.
MORAL? Even extreme feed-back forces felt on the controls may not indicate imminent breakup of your machine DO NOT PANIC!!! Look for the hydaulic switch (in differing cockpit positions depending on custom isation) and switch the damned thing into manual