A B737-700 IN CLB AT FL275 DIVERTED DUE TO A LOW FREQ HIGH AMPLITUDE AIRFRAME VIBRATION LASTING 2 MINS. CHANGE IN ENG THRUST OR AIRSPD HAD NO EFFECT.

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AS FLT WAS CLBING THRU FL275 A SUDDEN AIRFRAME VIBRATION OCCURRED. WE ADVISED ATC WE WOULD LIKE FL280. AT FL280; I REDUCED ENG THRUST AND SPD WHILE TRYING TO ISOLATE THE ORIGIN OF THE BAD VIBRATION. CHANGES IN ENG THRUST AND AIRSPD HAD NO EFFECT ON THE FREQ OR AMPLITUDE OF THE AIRFRAME VIBRATION (THE VIBRATION WAS A LOW FREQ MEDIUM AMPLITUDE). WHILE REDUCING SPD WE CHKED ALL SYS AND ALL SYS WERE NORMAL; FLT CTLS OPERATED NORMALLY. THE BAD VIBRATION LASTED ABOUT 2 MINS. AFTER WE CONTACTED DISPATCH AND MAINT; WE DIVERTED TO ZZZ AT 270 KTS (WE DEVIATED ABOVE 270 KTS AT ONE TIME). WE LANDED AT ZZZ WITHOUT INCIDENT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFO: THE RPTR STATED THE MAINT ACTION TAKEN TO CORRECT THE AIRFRAME VIBRATION IS UNKNOWN AS MAINT HAS RELEASED NO INFO TO THE RPTR. THE RPTR SAID THIS VIBRATION WAS OF A LOW FREQ AND HIGH AMPLITUDE LASTING 2 MINS. THE RPTR STATED THE VIBRATION WAS UNLIKE THE VIBRATION ASSOCIATED WITH AIRWORTHINESS DIRECTIVE 2002-08-20; WHICH IS A HIGH FREQ VIBRATION. THE RPTR SAID CHANGES IN THE AIRSPD AND ENG THRUST HAD NO EFFECT ON THE VIBRATION. THE RPTR STATED ON LNDG; A CONFERENCE CALL WAS INITIATED WITH THE CARRIER'S SYS ENGINEER; MAINT AND A BOEING ENGINEER AND THE VIBRATION WAS DISCUSSED. CALLBACK CONVERSATION WITH RPTR ACN 552694 REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CORRECTIVE ACTION TAKEN BY MAINT IS UNKNOWN. THE RPTR SAID IN PASSING THROUGH THE STATION RECENTLY THE RPTR DISCUSSED THE AIRPLANE WITH A TECHNICIAN AND WAS TOLD SOME PARTS WERE REPLACED IN THE TAIL. THE RPTR STATED THE VIBRATION WAS OF A LOW FREQ AND MUCH DIFFERENT FROM THE HIGH FREQ RELATED TO THE AIRWORTHINESS DIRECTIVE.