Richmond's mayor set out to solve the mystery and even offered a $500 reward to anyone that could identify the sound.
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"Ok, it literally sounded like dun, dun, dun, dun," Richmond resident Ruby Gorrostieta said.
This is the sound Gorrostieta still can't get out of her head.
"I didn't sleep, it was just in the back of my head, I would dose off for a few minutes but it kept me up," she said.
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Spanning from around 8 p.m. on Saturday night to 9 a.m. the next morning, she heard it while she was in the San Pablo area.
"It was really loud, it was kind of creepy because it just kept going and going and going, and I'm just sitting there like, this can only mean one thing, aliens," she said.
And she wasn't the only one.
Richmond Mayor Tom Butt says it echoed across Richmond, San Pablo and El Sobrante.
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"Nextdoor and Facebook were just lit up, literally hundreds of people all over West County were complaining about it, no one could figure it out, people were getting in their cars driving around trying to find it," Butt said.
So Mayor Butt posted a $500 reward on Sunday for anyone that could identify what the sound 'plaguing Richmond' was and where it was coming from.
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Tips started to roll in and two came from people attending an outdoor party in Richmond.
One tipster led the mayor to an empty industrial lot near Richmond Parkway and Goodrick Road.
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Beer cans, bottle caps, and speaker wires, a few of the only signs left from this party that was for the most part, kept under wraps.
Mayor Butt says it was a party with "electro-funk" style music.
"Apparently it's a cultural thing, where people get a pick-up truck and they pack the bed totally full of high-powered sound equipment and then they come together in some kind of a venue and they play all of this stuff at once," Butt said.
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ABC7 found a flyer for the event online, which appeared to be in Portuguese, before tracking down a man who claims to be the father of the organizer.
He says the whole thing was just his son's 20th birthday party.
"Everything they did was illegal," Butt said. "Richmond has a fairly robust noise ordinance, and doing something like that is illegal and it's punishable as a misdemeanor which is $1,000 or six months in jail."
The birthday boy's dad says he sent his son and his best friend, who also helped organize the party, straight to City Hall to talk with the mayor directly once he heard what happened.
The mayor's office confirms the men did come down but that the mayor was busy, so they did not meet.
ABC7 did call the Richmond Police Department to see if the young men might be facing any charges, but haven't heard back yet.