What was found
ON CLIMBOUT, THE AIRCRAFT EXPERIENCED A VIOLENT LATERAL FLUTTER. THE AUTOPILOT WAS OFF, YAW DAMPER ENGAGED. THE YAW DAMPER WAS DISENGAGED BY PILOT WITH NO NOTEABLE DIFFERENCE. DEPOWERED THE RUDDER HYDRAULICS AND THE FLUTTER STOPPED. FURTHER TROUBLESHOOTING VERIFIED THE RUDDER IN THE POWERED MODE CAUSED THE PROBLEM. ON PREFLIGHT, THE HYDRAULIC PRESSURE WITH AILERON AND RUDDER POWER OFF WAS 2920 PSI. WITH THE AILERON AND RUDDER POWER ON, THE PRESSURE WAS 2670 PSI. MAINTENANCE FOUND THE RUDDER HYDRAULIC POWER PACK BYPASSING FLUID. REPLACED THE RUDDER HYDRAULIC POWER PACK IAW DC8 MM 27-21-06.