AN A319 ACCELERATING ABOVE 300 KTS EXPERIENCED A MODERATE PITCH OSCILLATION THAT DISAPPEARED AGAIN AFTER DECELERATING BELOW 300 KTS.

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FLT DEPARTED FROM LAX RWY 24L AT APPROX XA15Z. DURING OUR INITIAL CLB WE ACCELERATED TO 250 KTS AFTER 1000 FT AGL AND WAS INSTRUCTED TO MAINTAIN 250 KTS UNTIL ADVISED BY SOCAL DEP. WE CONTINUED THE CLB TO FL230 AND WHEN REACHING FL230 WE WERE INSTRUCTED TO RESUME NORMAL SPD. WE CONTINUED THE CLB TO FL230 AND WHEN REACHING FL230 WE WERE INSTRUCTED TO RESUME NORMAL SPD. SINCE WE HAD BEEN DELAYED GETTING AIRBORNE DUE TO IN TRAIL SPACING; I ELECTED TO INCREASE THE SPD UP TO 340 KTS. AS THE ACFT ACCELERATED THROUGH 300 KTS; I BEGAN TO NOTICE A LOW PITCHED HUMMING NOISE SIMILAR TO THE NOISE A WING BODY FAIRING SEAL MAKES. AS THE ACFT ACCELERATED THROUGH 315 KTS; I BEGAN TO NOTICE A MODERATE FREQ OSCILLATION ABOUT THE ACFT LATERAL AXIS. I WOULD CHARACTERIZE IT AS A FLUTTERING TYPE OSCILLATION. IT WAS VERY APPARENT TO BOTH THE FO AND I THAT SOMETHING WAS NOT RIGHT. I BEGAN TO REDUCE THE SPD BACK BELOW 300 KTS AND THE VIBRATION CEASED BELOW 315 KTS. AS I WAS SLOWING THE ACFT; I RECEIVED A CALL FROM THE THIRD FLT ATTENDANT IN THE AFT GALLEY. SHE SAID SHE COULD FEEL A PRETTY GOOD THUMPING NOISE VIBRATING THROUGH THE FLOOR AND SHE WAS HAVING DIFFICULTY STANDING UP. I TOLD HER THAT WE WERE AWARE OF THE PROB AND TO LET ME KNOW IF THE THUMPING CONTINUED AFTER WE SLOWED DOWN. WE CONTINUED THE FLT TO LAS AT 285 KTS WITHOUT FURTHER INDICATIONS OF THE PREVIOUS VIBRATION. THE THIRD FLT ATTENDANT INFORMED US THAT AFTER WE TOLD HER THAT WE WOULD SLOW THE SPD OF THE ACFT THAT SHE DID NOT FEEL THE THUMPING ANYMORE. I INFORMED LAS LINE MAINT OF THE PROB AND FILLED OUT A DETAILED DESCRIPTION IN THE LOG. I ALSO COMPLETED AN AIRBUS VIBRATION RPT WHICH WAS GIVEN TO ME BY LAS MAINT. LAS MAINT PERFORMED SOME INSPECTIONS WITHOUT FINDING ANY PROBS BUT COULDN'T FINISH THE INSPECTIONS DUE TO LACK OF THE PROPER TOOLS. I WAS INFORMED THAT THE ACFT WAS TAKEN OTS PENDING A COMPLETE INSPECTION AND A TEST FLT.