Apple's top-secret Sunnyvale, Calif., automotive testing facility has been the source of loud "motor noises" late at night that irked at least one nearby neighbor, AppleInsider has discovered.

A resident who lives near Apple's automotive campus in Sunnyvale issued a complaint to city officials last year over concern about noise coming from the property. Major renovations at the Apple-occupied site have been in the works, including the addition of an "auto work area" and a "repair garage," AppleInsider was first to reveal last March.

Apple has built an "auto work area" and "repair garage" in the town of Sunnyvale, where one neighbor complained of "motor noises" late at night.

Whether the sounds were construction or something else remains unknown, as Apple has gone to great lengths to conceal what it is doing at the property.

"(Do) there have to (be) motor noises at 11:00 p.m. at night like last night?" the resident wrote to the city of Sunnyvale. "Even with the windows closed I could still hear it."

Apple occupies seven buildings, comprising nearly 300,000 square feet, of a large office complex in the city. One of the buildings, street number "175," was advertised as home to a mysterious company named SixtyEight Research, believed to be a shell corporation for Apple to conceal its real intentions.

Sources have indicated to AppleInsider that shipments related to the development of Apple's "Project Titan" automotive effort have been sent to its Sunnyvale campus, just minutes away from the 1 Infinite Loop headquarters. Specifically, "Project Titan" is said to be based out of a building known internally as "SG5."

Construction at Apple's Sunnyvale campus, as seen last year.

Whether the developments at the site are related to the advancement of the CarPlay platform or are a part of something larger — Â like a full-fledged Apple-built car — remains a mystery.

And Apple, of course, intends to keep it that way. Last year, representatives for the property received permits to build a 10-foot-tall security fence around the buildings Apple occupies, citing the need for both "physical security" and "visual privacy." The permits were required because the fence exceeds local height restrictions.

Numerous additional permits for construction inside the buildings specifically say the improvements were done "for Apple," leaving little question as to who requested the security fence, and who is responsible for the late-night noises.