Residents say the strange noise is keeping them awake at night.
Julie Schickling stood out on her porch in West Seattle, Washington just after midnight because she couldn't explain what she was hearing.
So she recorded the sound.
"It gets high and lower, and goes away, then comes back," said Schickling.
Some of her neighbors report being shaken out of bed by the low rumble, also described as a growl.
In fact, as many people you talk with is about how many different words you heard to describe it.
"It is kind of creepy," Kay Kirkpatrick, the West Seattle resident said of the sound. "It creeps you out a little bit."
The neighboring large industries say they aren't to blame.
The City of Seattle is looking into it.
Some long time residents say they've heard this sound before over the years.
Others say it's the first encounter they've had with the eerie noise.
"We want to know," Kirkpatrick said. "Tell us what it is."