What was found
AIRCRAFT WAS GROUNDED. AIRFRAME VIBRATION ON HORIZONTAL AXIS FROM 20,800 TO 28,000 AT 330 KTS. RUDDER INPUT STOPPED VIBRATION. REPLICATED SEVERAL TIMES. SPOILERS AND ELEVATORS HAD NO EFFECT. MAINTENANCE REPLACED RUDDER ATTACH FITTINGS NR 3 & NR 4, PN D5537131800200 AT RIB 5, RIB 6 AND RUDDER ATTACH FITTING NR 5 AT RIB 7, D5537131800000 IAW AMM 55-36-41-000-001-A. SEE AALA201606229011 FOR NR 3 AND NR 4 FITTINGS.