“It’s definitely a horrendous noise,” Dave Nemeyer, fire marshal of Forest Grove Fire and Rescue told ABC News. “I have no idea what the noise is. [The resident] described to us that it was coming from the middle of the street. To me, it sounds like the sound of train tracks, that metal screeching sound, but there are no train tracks near her home … so that’s obviously ruled out.”

Residents began hearing the strange noise, which lasts anywhere from a few seconds to several minutes, several weeks ago, according to ABC affiliate KATU. It is loud enough, residents say, that it rouses them from sleep and drives pets crazy.

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Paula Lynch told NBC affiliate KGW that she managed to record the sound on her cellphone earlier this month. She said it was the third time she’d heard the noise that week.

“My first instinct was that it was a gas line issue and there might be an explosion,” she said.

Worried about an impending disaster, Lynch reported the sound to police, who told the station that they’re also baffled.

“There would be a city ordinance violation if somebody was creating a noise like this, that late at night, on purpose to annoy people,” said Forest Grove Police Capt. Mike Herbs. “At this point, we don’t have information that would lead us to believe that’s the case.”

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“It sounds to me like some kind of release valve or some kind of pipe that’s under pressure,” he added. “We’ve had different suggestions from folks that it’s an alien mother ship or a warning sign of something to come.”

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Police aren’t the only ones at a loss to explain the noise. Both Northwest Natural, a gas company, and Forest Grove Public Works claim their agencies aren’t responsible for the sound, according to KGW.

After investigating, the fire department determined the sound was not coming from a commercial fire or smoke alarm, the station reported. The department, KGW noted, does not believe the noise poses a public health risk.

Authorities told KATU that the nearest train track dead-ends across town and is rarely used.

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Some residents wondered whether the sound could be coming from the brakes of a logging truck, but its duration and intensity are different, according to the station. There’s also the fact, KATU noted, that nearby roads are flat.

Although some have speculated that the sound is coming from a ruptured natural gas line, KATU reported that the lines are buried underground, making them an unlikely possible source. An unnamed spokesperson for Northwest Natural told KATU that a leaking gas line would sound like a tea kettle and that residents would smell gas.

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Another possible culprit, residents have speculated, is the Department of Forestry. The sound appears to originate in the agency’s vicinity, but after crews there tested their equipment on Tuesday, they concluded it wasn’t originating on their property, KATU reported.

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KATU asked Tobin Cooley, an audio expert, to weigh in. After measuring the noise with a sound meter, he told the station that it was a highly unusual case.

“It sounds like some sort of pressurized gas or air through a fitting or valve or something,” he said. “It’s not steady state, and you can’t predict when it’s going to happen. Those are all interesting sound features.”

Cooley noted that high-pitched tones don’t travel far, suggesting the sound is originating near the residents who are hearing it.