A320 CAPT REPORTS LOW FREQUENCY VIBRATIONS CLIMBING THROUGH FL260 AT 320 KNOTS. VIBRATIONS DISAPPEAR BELOW 290 KNOTS.

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DURING CLB THROUGH APPROX 26000 FT AT AN INDICATED SPD OF 320 KTS; ACFT STARTED LOW FREQ (APPROX 3-4 OSCILLATIONS PER SECOND); MODERATE INTENSITY VIBRATIONS THAT FELT VERY MUCH LIKE AERODYNAMIC BUFFETING. AT THE TIME OF THE ENCOUNTER; AUTOPLT #1 WAS ENGAGED. INITIALLY; I SELECTED A SLOWER SPD; HOWEVER; GIVEN THE INTENSITY OF THE VIBRATION; I WANTED TO INCREASE THE RATE OF DECELERATION. I THEREFORE DISCONNECTED THE AUTOPLT AND INCREASED THE ACFT'S PITCH IN ORDER TO INCREASE THE RATE OF DECELERATION. MY CTL INPUTS DID NOT SEEM TO AFFECT THE VIBRATION. THE VIBRATION CONTINUED UNTIL THE ACFT SLOWED TO APPROX 290 KTS. ONCE SLOWED; REENGAGED THE AUTOPLT. THE REMAINDER OF THE FLT WAS NORMAL AND UNEVENTFUL. WHEN I FIRST ARRIVED AT THE ACFT; I NOTICED THE LOGBOOK SHOWED 2 RECENT WRITE-UPS REGARDING VIBRATION. I ASSUMED THE VIBRATION DESCRIBED IN THESE LOGBOOK ENTRIES WERE THE HIGHER FREQ VIBRATIONS THAT HAVE BEEN NOTED MANY TIMES ON THE A320 FAMILY OF ACFT. THE VIBRATION ENCOUNTERED ON THIS FLT WAS MUCH; MUCH MORE INTENSE AND OF A MUCH LOWER FREQ. MAINT IS ADDRESSING THIS ISSUE.