First the good news: The mystery of the Sausalito hum may be just a fish story. Now the bad news: Earplugs may be the only solution.

The hum, likened to the drone of a B-29 bomber or a giant electric shaver, has caused a lot of sleepless nights in the houseboat colony in Sausalito, across the mouth of the bay from San Francisco.

Some residents, who recall the nocturnal hum reverberating through the cement hulls of their houseboats for more than a decade, have abandoned hull-bottom sleeping berths where the hum was loudest.

It was only in the past year that enough complaints prompted the Marin County Health Department to step in. Although there was no budget for investigating mysterious sounds, a series of volunteers have lent their feasible and not-so-feasible explanations for the noise.

But last week a new and promising theory arose.

``It's not a military secret,'' says John McCosker, director of the Steinhart Aquarium. ``It's not the sanitation district. It's not the Army Corps of Engineers. It's not an extraterrestrial, a nuclear device, or a Russian submarine.'' Most likely, he says, it's the singing toadfish, also known as the plainfin midshipman.

During mating season in summer -- which corresponds to the time houseboat dwellers usually complain -- the male toadfish burrows into the mud of estuaries and bays, then starts droning out for female companionship. The drone comes from muscle contractions around the swim bladder.

Dr. McCosker and other curious investigators rented a boat last week and, hovering over ``hot spots'' of sound, netted 10 of the singing toadfish. Though the fish in custody aren't talking so far, McCosker hopes they are males and that he can coax them into a few of bars of their monotonous tune so he can record a ``soundprint'' that might later be matched with the Sausalito hum.

``This is the best theory so far,'' says Frank Hubach, owner of an acoustical engineering company in nearby Berkeley, which has volunteered time over the past year to seach for the source of the hum. ``We always believed it was from a mechanical source; it doesn't sound like it's alive. But maybe we couldn't see the forest for the trees,'' he says.

Mr. Hubach has measured the hum at up to 40 decibels -- about 10 decibels above the ambient, or nondistinguishable, background noise. ``Usually, anything more than 5 decibels above the ambient is excessive,'' he explains, noting that the level of sound from the fish could be roughly compared to having a discoth`eque next door.

``The majority of people think fish are mute,'' says Thomas Niesen, professor of marine biology at San Francisco State University, who came up with the toadfish theory after reading newspaper accounts of the problem. That's why no one over the years would have thought to accuse fish for the racket under the Sausalito houseboats, he says.

``These fish congregate and, collectively, it's almost deafening [when heard underwater],'' Professor Niesen says.

Meanwhile, if the hum turns out to be the toadfish, the houseboat inhabitants' only relief is in knowing what's making the sound.