AN A320 CREW RPTS DETECTING LIGHT TO MODERATE AIRFRAME VIBRATIONS IN MID TO AFT CABIN. VARIOUS CAUSES WERE DETERMINED; INCLUDING OUT OF ROUND ELEVATOR SERVOS.

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ENRTE BTWN CITY A AND CITY B; A COMMUTING PLT SEATED IN THE MID CABIN CAME TO THE COCKPIT TO RPT THAT HE WAS 'CONCERNED' ABOUT VIBRATIONS IN THE CABIN DURING CRUISE FLT. HE SAID THE VIBRATIONS 'CAME AND WENT;' WERE LIGHT AND APPEARED TO BE ENG RELATED. HE INDICATED THE VIBRATIONS SEEMED TO BE LOCATED IN THE MID TO AFT CABIN AREA. THE FO WENT TO THE REAR OF THE CABIN AND CONFIRMED LIGHT VIBRATION. THE COMMUTING PLT DESCRIBED THE VIBRATIONS AS 'ENOUGH TO VIBRATE HIS LUNCH AND DRINKS OFF HIS TRAY TABLE.' VIBRATIONS WERE BARELY NOTICEABLE IN THE COCKPIT; ONLY OCCASIONAL LIGHT VIBRATION FELT THROUGH FLOOR AND RUDDERS. VIBRATION MONITORS IN BOTH ENGS WERE LESS THAN 1.0 ON #1 AND LESS THAN 2.0 ON #2 (NORMAL). THE COMMUTING PLT RPTED VIBRATIONS INCREASED ON DSCNT. THIS HAS BEEN A PROB ON THIS TYPE OF ACFT (A320). I HOPE YOU ARE KEEPING A GOOD DATABASE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HIS COMPANY HAS HAD A NUMBER OF VIBRATION RPTS AND LOGBOOK WRITE UPS. APPARENTLY; THE CAUSE WAS DETERMINED TO BE ASSOCIATED WITH THE AIRFRAME AND; IN 1 SPECIFIC CASE; 3 ELEVATOR SERVOS WERE FOUND TO BE OUT OF ROUND; CAUSING VIBRATION. ALSO; IN SOME CASES THE VIBRATIONS WERE EXPERIENCED ONLY AT HEAVIER WTS. PERHAPS RELATED - THERE HAVE BEEN A SERIES OF UNCOMMANDED ROLLS ON THE A320 ACFT IN VARIOUS PHASES OF FLT AND IN EACH CASE AN ELEVATOR AILERON COMPUTER FAULT WAS INDICATED. RPTR SAID HIS COMPANY SUSPECTS SPURIOUS SIGNALS IN THE R SIDESTICK; REMOVES THEM AND SENDS THEM BACK TO AIRBUS FOR EVALUATION. ADDITIONALLY; THE DIGITAL FLT DATA RECORDER INFO IS SENT TO AIRBUS. RPTR BELIEVES SENDING THE RECORDER INFO IS LIKE HAVING THE FOX GUARD THE HENHOUSE.