SAN FRANCISCO -- "Nails against the blackboard," said my SonicNet Music News colleague. He was predicting what the collaboration between Sonic Youth and Chicago-based post-rock guitarist Jim O'Rourke that I was about to witness Wednesday would be like. "Trust me."

The early afternoon performance took place at the cavernous Amoeba Records store in San Francisco, which occupies a former bowling ally located at the end of Haight Street, a few blocks from where the Grateful Dead lived in the mid-'60s. Some of the 300-plus SY fans in attendance -- purchase of the group's new album, A Thousand Leaves, at the store got you a ticket -- weren't so sure what they were going to experience. "It could be anything," said a fan named Dave who wouldn't provide his last name. "They might play songs or just destroy their guitars."

Not content to just play three nights of sold-out shows at the legendary Fillmore Auditorium, Sonic Youth set up some of their gear in the most appropriate of places -- a record store. This was an opportunity for the group both to jam with O'Rourke, with whom they collaborated on a recent EP, SYR 3, and to stretch out on more avant-garde -- and mostly instrumental -- material.

Before they began, O'Rourke, co-leader of Gastr Del Sol and a member of Oval, who was wearing a Spice Girls T-shirt, said that the performance would be completely improvised. "I haven't touched an electric guitar in two years, but that's probably what I'll end up playing today," he said. "I have no idea what we're going to do, we won't know until we get up there."

The music began quietly with SY drummer Steve Shelley trading grooves with a second percussionist who was beating out low notes on a kettle drum. Soon the duo was joined by SY guitarist Thurston Moore, who immediately began to coax feedback from his guitar.

As the rest of the musicians joined in, the music began to resemble the dull roar of a 747 taking flight. The relentless drone of three guitars grew more hypnotic as Moore, O'Rourke and SY guitarist Lee Ranaldo traded noisy licks while each stood motionless on the makeshift stage in the classical-music section of the store. Over the course of the show, the guitarists would manipulate their instruments with metal slides, drum sticks and nine-volt batteries.

The fans in attendance stood motionless with rapt attention. During some of the performance's quieter moments there was sporadic applause, which the musicians seemed oblivious to.

At times, the music reached a fevered pitch, with the syncopated cymbal hiss adding to the white noise of the three electric guitars. SY bassist/guitarist Kim Gordon sat on the chest-high stage for a portion of the show softly singing nonsense lullabies into a microphone while banging on the bass that lay onstage beside her.

After about an hour, the musicians began to leave the stage one by one as the cacophony subsided and was replaced by the quiet sounds of Moore and O'Rourke, who continued to unleash feedback for a few more minutes.

Complete silence finally signaled the end of the show, at which point the crowd exploded in enthusiastic applause.

John Sigmond, a fan who'd seen the band's second Fillmore show the night before, described the daytime show as "brilliant." "It's almost like their earlier stuff but with kind of a new edge," said Sigmond, 22. "To me, it's just really hypnotic, deep-thought music."

Back at the SonicNet office, my colleague was waiting for a report on the show. "So?" he asked.

"Nails on the blackboard," I told him.