AN A300 WITH SLATS EXTENDED ON APCH; OAT 5 DEGS C IN IMC WITH WING AND ENG ANTI-ICE ON; EXPERIENCED UNCOMMANDED ROLL OSCILLATIONS POSSIBLY DUE TO AIRFRAME ICE.

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THIS HAPPENED TO US WHILE WE WERE ON THE ILS RWY 15 AT ZZZZ. WE HAD BEEN VECTORED FOR THE ILS RWY 15 AND HAD BEEN IN THE CLOUDS FOR AROUND 5 MINS WITH AN OAT OF AROUND 5 DEGS C. WE WERE VECTORED FOR ABOUT 3 MINS WITH OUR SLATS EXTENDED AND AN INDICATED AIRSPD OF AROUND 210 KTS. AS SOON AS THE LOC WAS CAPTURED; THE ACFT WINGS STARTED TO ROCK. AT FIRST THEY WERE ROCKING JUST A LITTLE BIT AND GRADUALLY OVER A PERIOD THE ROCKING BECAME WORSE. WE WERE ROCKING ABOUT 20 DEGS EITHER SIDE OF VERTICAL. THE FO WAS FLYING AND I ASKED HIM TO TURN OFF THE AUTOPLT. WHEN HE DID THAT THE ACFT CONTINUED TO ROCK AND HE MENTIONED IT WAS DIFFICULT TO HOLD THE WINGS LEVEL. WE BUMPED THE SPD UP 5 KTS FROM THE REF SPD. WE BROKE OUT OF THE CLOUDS AT AROUND 500 FT AND CONTINUED VISUALLY. WE HAD A HARDER THAN NORMAL LNDG IN RELATIVELY LIGHT WINDS. AS SOON AS THE POWER WAS PULLED OFF THE ACFT QUIT FLYING. THIS WAS THE FIRST TIME THAT I HAVE FLOWN THE A300 AND HAD ACTUALLY SEEN ICE ACCUMULATION ON THE WIPER BLADES AND THE WINDSHIELD. ICING HAD BEEN RPTED IN THE CLOUDS BELOW 5000 FT. MY BEST GUESS IS THAT WE HAD ACCUMULATED A RATHER SIGNIFICANT AMOUNT OF ICE ON THE WINGS POSSIBLY BEHIND THE SLATS? THE ENG AND WING ANTI ICE WAS ON FOR OUR DSCNT AND APCH. THE ROCKING OF THE WINGS REALLY CAUGHT US OFF GUARD. THE AUTOPLT WAS NOT ABLE TO KEEP THE WINGS FROM ROCKING AND THE FLYING PLT COULDN'T STOP THEM FROM ROCKING FLYING MANUALLY.