Neighbors can't stop talking about a loud hum coming from Apple's mysterious labs

Apple labs in Sunnyvale, California have suddenly attracted a lot of attention from neighbors who say they hear loud noises coming from the facility.

Apple's not saying what it's working on there - signs point to the Apple Car - but whatever it is, neighbors can't stop talking about what might be causing loud noises behind a newly constructed sheet metal fence at night.

One neighbor described the sounds he hears at night as "sheets of metal slamming, clanking, almost a grinding sound."

Another told the San Jose Mercury News that he hears a loud "whine or hum that rises in pitch, like industrial machinery winding up."

One reason why Apple could be working at night is that electricity is cheaper then, and the hum could be coming from a new experimental electric charging system. MIT system dynamics lecturer John Carrier speculates that Apple could be trying to "surge-charge" a car's battery in seconds.

There's a lot of evidence that Apple is working on something car-related at its Sunnyvale labs.

• Apple met with representatives from GoMentum Station, a self-driving car testing ground, at the address last May
• Public permits indicate that there's an "auto work area" and "repair garage" as well as several large labs on-site
• A since-removed post on the Sunnyvale City Manager's blog in 2014 noted Apple was given a permit for an "auto testing facility."

Whatever's going on at the seven-building campus, it's unlikely to be revealed to the public for years. On Friday, CEO Tim Cook was asked about Apple's automotive ambitions, he replied:

"Do you remember when you were a kid, and Christmas Eve, it was so exciting, you weren't sure what was going to be downstairs? Well, it's going to be Christmas Eve for a while."