AN ACR FLC FLYING A B737-300 EXPERIENCES A HIGH FREQ VIBRATION AND UNCOMMANDED ROLL DURING CLB. THEY; WITH SOME DIFFICULTY; COUNTER THE ROLL; THE VIBRATION GOES AWAY AND THE FLC RETURNS TO THE ARPT.

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THIS INFO IS FOR DATABASE PURPOSES. INDICATING 235 KTS; CLBING THROUGH 4500 FT MSL WITH AUTOPLT ENGAGED; HDG MODE SELECT AND VERT SPD CLB WITH AUTOTHROTTLE; ACFT BEGAN TO VIBRATE/BUFFET/SHAKE. WX WAS VFR WITH LIGHT WINDS AND NO OTHER ACFT CLOSE BY. THEN ACFT BEGAN TO ROLL L. AS UNCOMMANDED ROLL CONTINUED; I SIMULTANEOUSLY DISENGAGED AUTOPLT AND AUTOTHROTTLE; PUT IN R AILERON AND ADDED PWR -- NO RESPONSE. I THEN TOLD THE FO TO TURN OFF THE YAW DAMPER WHILE ADDING FULL R AILERON AND FEEDING IN R RUDDER. ACFT SLOWLY; WHILE STILL VIBRATING AND SHAKING; BEGAN TO ROLL R TOWARDS WINGS LEVEL. ACFT WAS RETURNED TO DEP ARPT FOR UNEVENTFUL LNDG. CAUSE UNKNOWN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE VIBRATION ENCOUNTERED ON THIS FLT HAD BEEN BRIEFLY FELT DURING THE APCH ON THE FIRST FLT OF THE DAY. DURING THE APCH THE FO HAD THE 'B' CHANNEL OF THE AUTOPLT ENGAGED AND THE ACFT WAS IN THE APCH MODE AND ON THE ILS COMING DOWN THE GS WHEN A BRIEF EPISODE OF VIBRATION (1-2 SECONDS) WAS FELT WITH SLIGHT ROLLING MOMENTS TO THE L AND TO THE R AND THEN A RETURN TO NORMAL FLT. THE CREW WAS CONCERNED; BUT THE RPTR SAID THAT THE EPISODE WAS SO BRIEF THAT THEY COULD NOT DETERMINE IF IT WAS FROM SOME TURB OR OTHER SOURCE. THEY LANDED WITHOUT FURTHER DIFFICULTY AND PREPARED FOR THE SECOND LEG OF THE DAY AND THE PRIMARY EVENT. THE RPTR WAS FLYING THIS LEG AND AS THE ACFT LEFT 4500 FT; IN A SLOW CLB AT 550-600 FPM; THE HIGH FREQ VIBRATION BEGAN. THE ACFT WAS ON AUTOPLT; ON THE 'A' CHANNEL; AND HAD JUST ROLLED OUT OF A TURN TO A HDG OF 190 DEGS. AS THE VIBRATION CONTINUED; THE ACFT STARTED TO ROLL TO THE L; THE CAPT HAD PUT HIS FEET ON THE RUDDER PEDALS AND HIS HANDS ON THE WHEEL AND THE THRUST LEVERS BY THIS TIME. HE SAID THAT THE RUDDER PEDALS DID NOT MOVE AS THE ACFT STARTED TO ROLL. THE CAPT SAID THAT HE ATTEMPTED TO COUNTER THE ROLL BY USING THE AILERONS ONLY AND AFTER REACHING ABOUT 1/2 OF THE AVAILABLE INPUT WITH NO EFFECT HE REQUESTED THAT THE FO TURN OFF THE YAW DAMPER. THE REMOVAL OF THE YAW DAMPER HAD NO EFFECT SO THE CAPT USED ALL OF THE AVAILABLE AILERON AND ABOUT 1/4 OF THE AVAILABLE RUDDER TRAVEL AND THE ACFT SLOWLY RETURNED TO WINGS LEVEL. THE VIBRATION SLOWLY DECREASED THEN OVER A PERIOD OF 2-3 SECONDS. THE CAPT SAID THAT THE TOTAL PERIOD OF VIBRATION DURING THIS SECOND EVENT WAS 12-13 SECONDS. THE GROSS WT OF THE ACFT WAS APPROX 108000 LBS. AFTER THE VIBRATION CEASED THE FLC RETURNED TO DEP ARPT. DURING THE RETURN; ACCORDING TO THE CAPT; THE ACFT FELT UNSTABLE WITH THE NOSE MOVING WITHOUT ANY INPUT FROM THE FLC. THIS ANALYST ASKED IF THE MOVEMENT WAS SIMILAR TO SPOILER FLOAT; BUT THE CAPT HAD NEVER EXPERIENCED SPOILER FLOAT AND COULD NOT COMPARE THE MOVEMENT. HE DID SAY THAT THE VIBRATION WAS NOT SIMILAR TO A STALL. HOWEVER; HE SAID THAT HE HAS HAD EXPERIENCE WITH BOTH HIGH SPD AND LOW SPD STALLS AND THIS VIBRATION WAS OF MUCH HIGHER FREQ. THE RPTR SAID THAT HE DID PARTICIPATE IN A CONFERENCE CALL WITH COMPANY FLT DEPT; SAFETY; AND MAINT PERSONNEL AND HE DESCRIBED THE EVENT TO THEM AND LISTENED TO THEIR EXCHANGE. DURING THIS CALL; THE RPTR ALLEGES THE COMPANY SAFETY SAID THAT THERE HAD BEEN APPROX 40 RPTS OF UNCOMMANDED ROLLS IN THE PAST YR WITH A NUMBER OF THEM ACCOMPANIED BY THIS VIBRATION PHENOMENON. THE RPTR DID NOT KNOW IF THIS WAS A COMPANY TOTAL OR AN INDUSTRY TOTAL OF RPTS.