A B737-300 IN CRUISE AT FL330 HAS A MAINT CTLR REQUEST CREW TO SHUT OFF HYD PRESSURE TO THE FLT CTLS FOR TROUBLESHOOTING.

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ACR FLT #XXX LAS-IAH FEB/XA/01. DEPARTED XA42; ARRIVED XC33. ACFT #XYZ; B737-300. THIS STATEMENT IS IN REGARD TO AN INCIDENT THAT OCCURRED DURING FLT ABOUT 40 MINS AND 150 NM N OF PHOENIX AT FL330. I WAS THE FO ON THIS FLT AND IT WAS DESIGNATED AS MY LEG TO FLY. AFTER ROTATION; THE CAPT AND MYSELF NOTICED A LOW FREQ. IT DID NOT SUBSIDE AFTER GEAR RETRACTION. THE CAPT INDICATED HE HAD PREVIOUSLY FLOWN THIS AFT AND EXPERIENCED A SIMILAR VIBRATION. DEP AND CLBOUT WERE COMPLETELY NORMAL. ONCE ENRTE; THE CAPT INDICATED HE WOULD CONTACT PHX LINE MAINT VIA ACR NET WHILE I CONTINUED TO FLY AND MONITOR CTR FREQ. I DID NOT LISTEN IN ON COM #2 WITH THE CAPT AS CTR FREQ WAS STILL QUITE BUSY. AFTER A FEW MINS; THE CAPT STATED THAT MAINT INSTRUCTED US TO REMOVE HYD PWR BRIEFLY TO SEE IF THE VIBRATION STOPPED; THUS GIVING THEM AN INDICATION OF A POSSIBLE LOOSE TRIM TAB. I ASSUMED THE CAPT AND MAINT WERE FOLLOWING A PRESCRIBED CHK AND THAT THEY WERE FULLY AWARE OF ALL POSSIBLE RAMIFICATIONS OF SUCH A CHK. THE CAPT ASKED ME TO HAND FLY WHILE HE REMOVED HYD PWR. ONCE REMOVED; THE ACFT MADE NO ABRUPT MOVES. IT WAS DETERMINED THAT THE VIBRATION DID NOT SUBSIDE AND THE ACFT BEGAN A VERY SLIGHT TENDENCY TO BANK R AT APPROX .5 DEGS PER SECOND. I BEGAN A L INPUT OF AILERON; BUT WAS UNABLE TO CORRECT THE FLT PATH. I ASSUMED THE MANUAL REVERSION WAS SLUGGISH AND INCREASED THE INPUT WITH NO RESULT. I WAS ABOUT TO ADD L RUDDER AS WE DECIDED TO REPWR THE HYDS; SO I CTRED THE CTLS. PWR WAS RE-ESTABLISHED AND I CORRECTED THE 3-4 DEG BANK; THEN SQUARED THE FLT PATH WITH THE FLT DIRECTOR AND RE-ENGAGED THE #2 AUTOPLT. THE FLT PROCEEDED TO DEST NORMALLY AND ARRIVED AT XC40 LCL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS NOT HAPPY WITH THE CAPT'S ACCEDING TO THE MAINT CTLR'S REQUEST TO TURN OFF 'A' AND 'B' HYD PWR TO THE FLT CTLS FOR TROUBLESHOOTING A VIBRATION. THE RPTR SAID THIS REQUEST WAS NOT IN THE INTEREST OF SAFETY FOR THE PAX AND CREW.