In Sunnyvale, California, what goes bump in the night is a top-secret Apple facility. Cryptic noises are awakening neighbors of the 96,000-square-foot former Pepsi bottling plant, who, along with reporters, are beginning to ask questions. Apple is staying quiet — for now.

It’s a classic mystery tale with a Californian twist. Undercover security guards obscuring their shirts’ logos, waylaying innocent passersby. Those same sentinels telling dog-walking neighbors to buzz off. White Priuses quietly tailing afternoon strollers. Inquisitive homeowners and tight-lipped spokespeople.

Apple erected twelve-foot sheet-metal fences to enclose and obscure the facility’s operations. But come nightfall that obscurity no longer matters: Neighbors say that that’s when the ambiguous, unsettling, and at times jolting noises start. The strangest sound, these neighbors say, is “a whine or hum that rises in pitch, like industrial machinery winding up.” The San Jose Mercury reports:

“‘It’s almost like they want to do it under cover of night,’ says … James Porter, a 48-year-old operations manager for an electronics company.

“A few doors down from the Porters, home-construction contractor Jamie Rouleau, 58, remains puzzled by a noise he has heard coming from Apple that sounds like someone “waving around” a large piece of sheet metal. Bangs and thumps, as well as the beeping of reversing trucks resound in the middle of the night, Porter said.”

The former Pepsi bottling plant that now seems to have more interesting things going on inside.

Vague permit applications for “auto work areas” and a “repair garage” do well to fuel rumors that this Apple facility is a workspace for Project Titan. In other words, it’s a place for Apple engineers to retire to R&D the heck out of whatever futuristic, electric, and/or self-driving vehicle the once-tech company has in the works. The rumor mill has long been spouting rumors that Sunnyvale is the Project Titan hub, and a mostly-non residential area would be as good a place as any for such a project.

The Mercury reports that we may be able to chalk up the witching-hour work schedules to cheaper, off-peak electricity costs, and a MIT lecturer speculates that the intensifying, droning noise — the most mysterious and jarring sound of the whole racket — could be a “high-intensity electrical capacitor.”

Elon Musk, who may be a bit sidetracked at the moment, calls this project Apple’s “open secret.”

That secret may continue to unravel if Apple keeps waking the locals.