The Sunnyvale campus has been linked to Apple through city documents, and there's a fair amount of evidence that the property is where Apple is developing its car that has been called an "open secret."
"[Do] there have to [be] motor noises at 11:00 p.m. at night like last night? Even with the windows closed I could still hear it," the Sunnyvale resident wrote, according to Apple Insider.
It's possible that the resident may be mistaking construction sounds for the revving of an engine. Last year, the city of Sunnyvale issued permits to construct a "windowless repair garage" at one of the buildings Apple operates.
But previous reports have indicated that Apple is doing prototyping work at the facility. If Apple were to be building, say, a car frame or automotive components, the lathes and mills needed to shape the metal could produce a fair amount of noise as well.
Also, in the past week, Apple posted a job opening for a "welder/fabricator" in Santa Clara county under its hardware engineering department. The listing asks for large-scale prototyping skills, including experience with TIG and MIG welding, and the ability to bend tubes.