B747 Captain was informed by the cabin crew of banging noises during takeoff; similar to loose cargo containers. The same noise occurred during landing and post flight revealed three unsecured containers and two empty spaces allowing movement.

Narrative

At about 2000 FT the Relief Pilot answered an interphone call from the cabin. The Purser was reporting that the two flight attendants at doors 2 left and right reported loud banging noises at power-up and rotation below their seats that sounded like cargo containers banging into each other. The First Officer flying said that he did not feel any unusual change in pitch. After a brief discussion concerning the safety of continuing the flight; we concluded that any shift in the center of gravity had already occurred; and it would be safe to continue. In the meantime we contacted Dispatch with our information. They informed us that two empty spaces had been vacant in the forward bay. We concluded that anymore shifting might occur upon landing at brake application. After landing roll; the Purser called to inform us that the same noise occurred below doors two left and right after landing. In flight we had asked to have the cargo bay inspected upon arrival. Before leaving the aircraft; I was met by a Mechanic that debriefed me that three cargo bins had been found unsecured in the forward cargo bay.