What was found
ACFT LAUNCHED FROM BASE, 13 MIN FLT WAS UNEVENTFUL, GW WAS 2754 & POWER WAS NORMAL. WHEN LIFTING FROM LOW ROOFTOP PAD, ACFT REQUIRED MORE TORQUE THAN EXPECTED TO HOGE, ASSUMPTION WAS HIGHER THAN EXPECTED WEIGHT BUT POST FLT DETAILED ANALYSIS: GW WAS 2778, OAT 10 DEG C, DA OF -650'. MAX CONTINUOUS POWER 69% HOGE SHOULD HAVE BEEN 2835KG, BUT REQUIRING APPROX 73-75% WITH A COUPLE MOMENTARY EXCURSIONS TO 75-78% TO ACHIEVE A HOVER AT 15-20'. ON TRANSITION, ACFT CONTINUED TO FEEL POWER LIMITED BUT MANAGED CLIMB OUT. AT APPROXIMATELY 60 KIAS, A SIGNIFICANT LATERAL VIBRATION, EXCURSION WAS FELT. VIBRATION A DEFINITIVE LATERAL FORCE IN A SINGLE SHARP MOTION EACH TIME. A FEW MOMENTS LATER, SAME VIBRATION FELT & CONTINUED AT RANDOM INTERVALS. LEVELED OFF AT 800', 80 KIAS & CHKD OBSERVING THAT THERE WERE NO ABNORMAL GAUGE INDS, WARNINGS OR CAUTIONS. ELECTED TO TURN TO NEARBY AIRFIELD. IN ADDITION TO LATERAL VIBS BEGAN TO EXPERIENCE SIMILAR FORCE VERTICAL VIBS, AS 3 OR MORE UNCOMMANDED ROTOR DISK PITCH EXCURSIONS OF +/- 15 DEGREES. SLOWED BELOW ETL, VIBS CONTINUED TO INCREASE & WERE NOT INDIVIDUALLY DISCERNIBLE BTWN VERTICAL & LATERAL BUT ACFT BECAME BARELY CONTROLLABLE BEFORE LANDING.