Hi HJ,
While I haven't had the same problem, it brings back to mind a quite related experience on the 109C. 1st symptom encountered was a vertical vibration (med freq) whether coupled/decoupled-- more of periodic and not persistent, but happens unpredictably. No stick feedback. Sure it had the tech dept scratching heads and couldn't find the cause. Even replaced all M/R rod ends, changed dampers, checked Hyd Sys, to no avail. We were maintained by Agusta branch.
One day we decided to reproduce and just stayed on hover and watched all instruments. Voila, the #2 Art. Horizon (not ADI) vibrates in pitch just as the a/c vib comes on. Finally, we've localized the problem with the art. horizon. I was told that part of the SAS (one of 2) is wired to the #2 display and the SAS is correcting the error. Replaced & problem resolved. Installation is a Sperry System.
In another a/c (AS350B), experienced cyclic feedback while flying offshore-- no pressure loss on Hyd Sys, no caption light. Suspected valve problem inside Servo and continued flight "isolated". Whew! drained all my energy! Replaced lateral servo and problem resolved.
Hope this helps.
b@6