Marine biologists say a beluga whale which was recorded making human-like noises in a US aquarium may have been trying to communicate with its keepers.

The whale, known as NOC, had lived among dolphins and other beluga whales in an aquarium in San Diego, California, and was often in the presence of humans.

Scientists who were puzzled by the human-like utterances coming from his pool concluded they were actually coming from the whale himself.

An acoustic analysis of the sounds showed a rhythm similar to that of human speech, Sam Ridgway, of the National Marine Mammal Foundation, said.

The frequencies were also several octaves lower than typical whale sounds.

"Such obvious effort suggests motivation for contact," Mr Ridgway said.

"The sounds we heard were clearly an example of vocal learning by the white whale."

Mr Ridgway is the chief author of a study of the case which has been published in the journal Current Biology.

The whale seems to have made an extraordinary effort to imitate humans. Experts say he would have had to modify his vocal mechanics by varying pressure in his nasal tract to produce this remarkable sound.

NOC died five years ago but had made human-like sounds for around four years until he reached the age of sexual maturity, Mr Ridgway said.

Mr Ridgway said that in 1984 he and others started hearing sounds near the whale and dolphin enclosure that sounded like two people speaking in the distance, too far away to be understood.

The sounds were later traced to NOC when a diver in the whale's tank came to the surface because he thought he heard colleagues tell him to do so.

ABC/AFP