US marine biologists puzzled by human-like sounds coming from the whale and dolphin tank of an aquarium concluded they were actually coming from a whale.

Anecdotal reports of whales sounding like people are not new. But in this case in San Diego, California, scientists for the first time recorded the utterances, did an acoustic analysis and were surprised to find a rhythm similar to that of human speech, Sam Ridgway of the National Marine Mammal Foundation reported on Monday.

The sounds marked quite a feat: Whales make sounds via their nasal tract, unlike people, who use their larynx. So this particular white whale had to make some tricky muscular and blowhole adjustments.

"Such obvious effort suggests motivation for contact," said Ridgway, the main author of a study featured in the journal Current Biology. "The sounds we heard were clearly an example of vocal learning by the white whale."

The beluga whale, named Noc, died five years ago.

Ridgway says that back in 1984, he and others started hearing sounds near the whale and dolphin enclosure that recalled two people speaking in the distance, too far away to be understood.

The sounds were later traced to one particular white whale when a diver in its tank came to the surface because he thought he heard colleagues tell him to do so.

Noc had lived among dolphins and other white whales and had often been in the presence of humans.

The whale made human-like sounds for around four years until it reached the age of sexual maturity, Ridgway said.