GREENVILLE COUNTY, S.C. —

A "mystery sound" heard by hundreds of people throughout the Upstate has caused a stir on social media platforms like Reddit and NextDoor, many asking the same question: where is it coming from?

One Greenville County neighbor, Laurie Smith, says she heard it from 11 p.m. to midnight on Thursday.

"Basically, it sounds like there's a jet engine right in your backyard!" said Smith.

The reality isn't too far off, either. What neighbors heard Thursday night was confirmed to be engine tests run by Lockheed Martin at Donaldson Center.

"It kind of rattles you out of bed, it's absolutely the loudest sound, said Smith. "That would get me right out of bed, thinking I'm being attacked!"

While some online users expressed concerns over the late-night timing of the tests,

See Full Page