In this specific instance, it is a Beluga whale named Noc. Apparently when Noc started doing this it fooled a diver in the pool at the time into thinking that someone was shouting at him to get out of the water. The Guardian reports …
Handlers at the National Marine Mammal Foundation in San Diego heard mumbling in 1984 coming from a tank containing whales and dolphins that sounded like two people chatting far away.
It wasn’t until one day, after a diver surfaced from the tank and asked: “Who told me to get out?” that researchers realised the garble came from a captive male Beluga whale.
Wait what … 1984 … but why is it making news now? Well because an article has just today been published in Current Biology journal about it all entitled “Spontaneous human speech mimicry by a cetacean” (and you will be delighted to know that there is no paywall there, so you can click on the right-hand side of the link to go here and read it all)
Are you screaming, “enough of all this sciency stuff I want to hear the whale talk“, well fret not, because here it is …
Abstract for the Paper
For those that are curious to find out a bit more, here is the paper’s abstract …
Although dolphins (Tursiops truncatus) have been trained to match numbers and durations of human vocal bursts  and reported to spontaneously match computer-generated whistles , spontaneous human voice mimicry has not previously been demonstrated. The first to study white whale (Delphinapterus leucas) sounds in the wild, Schevill and Lawrence  wrote that “occasionally the calls would suggest a crowd of children shouting in the distance”. Fish and Mowbary  described sound types and reviewed past descriptions of sounds from this vociferous species. At Vancouver Aquarium, Canada, keepers suggested that a white whale about 15 years of age, uttered his name “Lagosi”. Other utterances were not perceptible, being described as “garbled human voice, or Russian, or similar to Chinese” by R.L. Eaton in a self-published account in 1979. However, hitherto no acoustic recordings have shown how such sounds emulate speech and deviate from the usual calls of the species. We report here sound recordings and analysis which demonstrate spontaneous mimicry of the human voice, presumably a result of vocal learning , by a white whale.
… and if that tempts you to read even more, then here is the link to the paper.
Finally, there is a bit of sad news here as well, Noc is no longer with us. After four years of copying people, he reached maturity and apparently either lost the capacity to make human noise or lost interest in doing so. He went back to sounding like a whale, emitting high-pitched noises, and died five years ago.