Humpback whales are renowned crooners, belting out soulful songs that have come to symbolize not just their own intrigue, but deep mysteries of the ocean in general.

These songs have entranced humans for decades, ever since they were first recorded by U.S. scientists in the 1960s. They even went multi-platinum with the 1970 album "Songs of the Humpback Whale," which helped transform the animals' public image and remains the best-selling nature album of all time.

And now a very different, low-frequency sound has been recorded among humpbacks wintering in Hawaii, raising new questions about the whales' social dynamics. The sound was first heard in 2005 by Jim Darling, a research biologist with Whale Trust Maui, but it took him years to capture high-quality recordings.

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"Imagine hearing a heartbeat-like sound in the ocean but not knowing the source," the nonprofit group says in a statement. "Whale Trust researchers spent a decade listening for these sounds and wondering what they were. Finally, on a calm glassy day these sounds were recorded within a few meters of a pair of humpback whales."

The "pulse trains" were recorded near Maui, where some 10,000 humpbacks migrate from Alaska every winter to breed, give birth and nurse. They typically occur at a frequency of around 40 hertz (Hz), according to Whale Trust. Human hearing ranges from 20,000 to 20 Hz, so they're just barely audible to us.

You can hear a sample in the audio clip below; listen closely for a heartbeat-like noise in the background, behind more familiar-sounding whale songs:

These sounds are deeper than any confirmed humpback call, and as Darling tells National Geographic, he didn't think he was listening to an animal at first. He initially looked for passing helicopters and "then started wondering about submarines," he says, adding that "whales were way down the list."

Darling still can't be 100 percent certain whales are making this sound, although he says that's the most likely explanation. For one thing, the two humpbacks were the closest known suspects when the beat was recorded. "Even more convincing," Whale Trust notes in its statement, "the sounds increased in volume as the whales got closer and became softer as the whales swam away."

Other animals are known to produce low-frequency sounds beyond the range of human hearing, including large land mammals like elephants. Blue whales and fin whales also emit pulses at frequencies similar to the new recordings, but this would be the first evidence of anything like this coming from humpbacks.

humpback whale breachingHumpbacks travel more than 3,500 miles every year from Alaska to spend winters in Hawaii. (Photo: Shutterstock)

Even if humpbacks are responsible for these beats, it's too early to speculate about their purpose. But as Darling points out in his study about the discovery, they were recorded during breeding season when both males and females were present, raising the possibility that female humpbacks aren't as quiet as we thought.

"Is it part of the extensive male repertoire of sounds," he asks rhetorically in the study, "or is it female communication in an acoustic niche avoiding the high male-generated noise levels of the winter assembly?"

Only time (and more research) will tell, but Whale Trust Maui is optimistic about the sounds' scientific significance. "If verified," the group writes, "and these sounds are indeed another communication channel for humpback whales beyond the familiar song and social sounds, it could completely change how we view and interpret whale behavior in the breeding grounds."