What was found
HIGH FREQUENCY VIBRATION STARTED 23,500- 330 KNOTS WENT AWAY 24,500-300 KNOTS RETURNED AGAIN 25,000-315 KNOTS AND STOPPED 30,500-290 KNOTS .76 MACH. VIBRATION VERY NOTICEABLE THROUGHOUT CABIN. FLT CONTROLS SEEMED OK. CREW REPORTED UP & DOWN OSCILLATION THROUGHOUT AC. [SUP 06-01-07] PERFORMED ELEVATOR FREEPLAY AND OSCILLATION CHECKS. RT ELEVATOR SERVO VALVE FAILED OSCILLATION TEST AND WAS REPLACED.