A B737-300 IN CRUISE AT FL330 EXPERIENCES ACFT VIBRATION. CONTACTED MAINT CTLR WHO REQUESTED TURNING OFF FLT CTL HYD PWR FOR TROUBLESHOOTING.

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EXPERIENCED LOW FREQ AIRFRAME VIBRATION AFTER LIFTOFF AND THROUGHOUT CLB TO FL330 AND CONTINUED AT FL330. TURNS AND PWR CHANGES DID NOT ALTER THE VIBRATION. COULD BE FELT THROUGH RUDDER PEDALS; YOKE; PWR QUADRANT; AND SEAT OF THE PANTS. FLT CONDITIONS WERE CLR AND SMOOTH. CONTACTED MAINT CTL APPROX 30 MINS INTO THE FLT VIA COM RADIO AT APPROX 125 MI N OF PXR. AFTER ADVISING MAINT CTL OF THE SIT AND ANSWERING SEVERAL OF THEIR QUESTIONS; MAINT CTL PUT US ON 'HOLD' TO DISCUSS/RESEARCH THE SIT. AFTER A FEW MINS PASSED; MAINT CTL ASKED US TO REMOVE HYD PWR FROM THE FLT CTLS AND NOTE ANY CHANGES IN THE AIRFRAME VIBRATION. I ILLUMINATED THE SEATBELT SIGN AND HAD THE FLT ATTENDANTS SECURE THE CABIN AND SEAT THEMSELVES. WITH THE FO AT THE CTLS AND GUARDING THE RUDDER AND YOKE; THE AUTOPLT WAS DISENGAGED; ACFT WAS TRIMMED; AND HYD PWR WAS REMOVED. NO VIBRATION CHANGES WERE NOTED; HOWEVER; THE ACFT BEGAN A SLOW R ROLL (2-3 DEGS PER SECOND) REQUIRING L AILERON INPUT TO MAINTAIN STRAIGHT AND LEVEL. AS MORE L AILERON WAS REQUIRED TO SLOW THE R TURN; WE SIMULTANEOUSLY CTRED THE CTLS AND RESTORED HYD PWR TO THE FLT CTLS RETURNING TO NORMAL FLT CONFIGN. RESULTS WERE DISCUSSED WITH MAINT CTL VIA THE COM RADIO AND APPROPRIATE MAINT LOGBOOK ENTRIES WERE MADE. THE FO AND I NEVER FELT WE HAD DONE ANYTHING UNSAFE. WE DID WHAT MAINT CTL ASKED US TO DO; AND WE RELIED ON THEIR 'SAGE' ADVICE. I WAS SUBSEQUENTLY ADVISED BY AN ACR FLEET MGR THAT MAINT CTL SHOULD NEVER HAVE ASKED US TO REMOVE HYD PWR -- THAT SORT OF 'TROUBLESHOOTING' WAS LIMIT TO POST-MAINT FUNCTIONAL CHK FLTS.