AN A320 PILOT REPORTS AN AIRFRAME VIBRATION AT FL280/M0.78/300 KTS WHILE CLBING. AIRSPEED INCREASED VIBRATION AND IT APPEARED ASSOCIATED WITH THE RUDDER.

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CLBING AND ACCELERATING WE STARTED TO NOTICE AN AIRFRAME VIBRATION. IT WAS A CONSTANT FREQ VIBRATION THAT WAS BEST DESCRIBED AS DRIVING A CAR OVER A WASHBOARD. WE LEVELED OFF AT FL280; 300 KIAS/.78 MACH; AND SET UP FOR A VIBRATION CHK; WORKING EACH FLT CTL AXIS INDIVIDUALLY. RUDDER INPUT MADE THE VIBRATION STOP. ENG PARAMETERS WERE NORMAL; AND THE VIBRATION WAS NOT ASSOCIATED WITH ENG RPM AND WASN'T ANYWHERE NEAR THE FREQ THAT YOU GET WITH AN ENG VIBRATION PROB. AIRSPD WAS A FACTOR IN VIBRATION AMPLITUDE. THE FASTER WE WENT; THE LARGER THE VIBRATION; AND CONVERSELY; THE SLOWER WE WENT; THE LESS THE VIBRATION. WE SLOWED TO 250 KIAS AND MAINTAINED THAT UNTIL IN THE PATTERN BACK AT ZZZ. DURING THAT TIME; BOTH PLTS COULD FEEL THE VIBRATION IN THE RUDDER PEDALS; BUT NOT IN THE AIRFRAME. IN THE PEDALS; IT FELT TO ME LIKE IT WOULD SHAKE A LITTLE AND THEN BUILD TO 1 LARGER AMPLITUDE POP; THEN BACK TO THE SMALL SHAKES. NOW; DON'T GET ME WRONG HERE; THE PEDAL SHAKING WAS VERY SMALL; BUT IT WAS NOTICEABLE. WE BOTH EXPERIMENTED BY PUTTING OUR FEET ON THE FLOOR; WHICH WAS VERY SMOOTH; AND THEN ON THE PEDALS; WHICH BROUGHT ABOUT THAT SMALL VIBRATION. WHILE WE WERE AT ALT AND TROUBLESHOOTING THE PROB; WE CONTACTED MAINT AND HAD THEM PATCH US THROUGH TO DISPATCH. DURING THAT TIME; BOTH THE FO AND I CONFIRMED THAT IF WE HELD STILL AND STARED AT THE PFD AND NAV DISPLAY; IT APPEARED THAT THEY WERE VIBRATING BACK AND FORTH LATERALLY; THAT IS; IN YAW. SINCE THEY ARE BOLTED INTO THE AIRFRAME; WE CONCLUDED THAT IT WAS WE PLTS WHO WERE BEING BOUNCED L AND R SLIGHTLY; AND THAT FOR US THIS WAS ANOTHER CONFIRMATION THAT IT WAS A RUDDER FLUTTER PROB. WITH DISPATCH AND MAINT WE DISCUSSED OUR OPTIONS (CONTINUE; RETURN TO ZZZ; OR LAND AT THE NEAREST SUITABLE). I FELT IT WAS IN THE BEST INTEREST OF THE COMPANY THAT A RETURN TO ZZZ WAS BEST; AND MAINT CONCURRED. DISPATCH WANTED TO KNOW IF IT WAS SOMETHING WE NEEDED TO LAND ASAP FOR; AND IT WAS MY AND MY FO'S OPINION THAT AS LONG AS WE STAYED AT 250 KIAS OR LESS; IT WAS JUST FINE TO FLY IT BACK TO ZZZ. THAT WAS THE PLAN AND THAT'S WHAT WE DID. I INFORMED THE FLT ATTENDANTS; AND DID A CABIN ADVISORY BRIEFING WITH THE PURSER. I INFORMED THE PAX OF OUR PROB AND TOLD THEM OF OUR RETURN TO ZZZ. THE APCH; LNDG AND TAXI-IN WAS UNEVENTFUL.