PACIFIC GROVE – It’s “odd,” “disturbing” and keeping Pacific Grove residents up at night.

The droning hum emanates from the Monterey Bay and has baffled residents all summer long. One reader describes it as a “nighttime engine sound,” while another says the hum has led to several sleepless nights. But rest assured, two experts are silencing theories of military experiments and malfunctioning sewer pumps. In fact, they say it’s most likely the sex call of a species known as plainfin midshipman — or a toadfish.

The sound is being heard by residents living about a quarter-mile away from the Monterey Bay Aquarium. Jim Covel, the aquarium’s director of guest experience — who has experience with this species — said the sound likely comes from “lonesome males” trying to attract mates.

The endless motor-like sounds are produced by their strong muscles pressing against their balloon-like bladders. They hit the bladder about 6,000 times a minute — twice the speed of a hummingbird’s wings — and do it during a mating season that usually starts in May and ends in September. The loud hum can last for more than an hour at a time.

But Covel says the fish aren’t big. In fact, they are about six to seven inches wide.

Senior aquarist, Steve Brorson, who used to work at the Monterey Bay Aquarium for 25 years, said he owned one of these toadfish in the past, and just around summertime when mating season began, he had to release him because he couldn’t handle the hum.

“After three nights of the grunt-like hum I couldn’t do it anymore,” Brorson said. “I couldn’t even watch a TV show at medium volume.”

Covel said the sound described by readers could only be a possible explanation for those who live nearby the ocean, especially if they live near the areas with sandier or muddier water.

If they don’t, the humming toadfish may not be the answer to your mystery.