What was found
UPON TAKEOFF, ONCE AIRBORNE RUDDER BEGAN YAWING LEFT AND RIGHT AT ABOUT 1 HZ. CONTINUED YAWING LEFT RIGHT LEFT RIGHT THE ENTIRE DURATION OF THE FLIGHT. LEVELED OFF AT 8000 FEET AND SLOWED TO 230 KN AND CONTINUE TO BRING THE SPEED BACK TO TRY TO KEEP AERODYNAMIC FORCES OFF OF THE VERTICAL STABILIZER AND RUDDER. TRIED TO PUT PRESSURE ON THE RIGHT RUDDER MANUALLY DID NULL THE OSCILLATIONS. TRIED THE LEFT RUDDER DID NOT AFFECT THE OSCILLATIONS. OSCILLATIONS CONTD WITH THE LEFT AUTO PILOT ENGAGED, DISENGAGED THE AUTO PILOT STILL NO EFFECT. SELECTED VNAV, SPEED, FLIGHT LEVEL CHANGE AND VERTICAL SPEED NO CHANGES. CONFIGURED THE FLAPS 1 AT 200 KN, NO CHANGE. FINALLY ON FINAL FULLY CONFIGURED AT 150 KN WAS ABLE TO PUT PRESSURE ON A RUDDER NULLED OUT THE ISOLATIONS. LANDED SAFELY. NO EICAS THROUGHOUT THE EVENT. NO WARNING LIGHTS, NO YAW DAMPER, RUDDER RATIO, ETC. NO INDICATIONS OF CAUSE OF FAILURE. ***EMERGENCY DECLARED***