SUNNYVALE — Jamie Porter is standing on the roof of her house in a working-class neighborhood near Central Expressway, looking over the top of a green, sheet-metal fence to a complex of buildings just beyond. What’s going on in those buildings is a secret — the fence is tall, security is tight and Apple isn’t saying.

Unfortunately, Porter’s rooftop view sheds no light on the mystery: “Just buildings, and there’s a big steel box; otherwise the fence is way too high,” says the 19-year-old delivery driver.

This is the home of Apple’s latest mystery — a complex on San Gabriel Drive near Wolfe Road that has been occupied by the secretive Cupertino company since 2014, city records show. And whatever Apple is working on inside — a dreamy new car, some clues suggest — has over the past several months become the talk of this neatly trimmed slice of Silicon Valley, producing odd and telling noises, sometimes long after darkness has fallen.

“It’s almost like they want to do it under cover of night,” says Porter’s father, 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, James Porter said. “When you’re asleep and you hear that bang or thump, you kind of jump,” he said.

The Porters and Rouleau describe the strangest sound emanating at times from Apple’s complex as a whine or hum that rises in pitch, like industrial machinery winding up. The noise is “loud enough to wake you,” Rouleau said.

And then there is the security, heavy and covert, according to neighbors. The Porters said that whenever they walk their dogs Harley and Charlie on San Gabriel, white Prius cars follow them. Usually, when the Porters get about halfway down San Gabriel, they’re “intercepted” by security, Jamie Porter says. The guards, as they approach, use clipboards to cover logos on their shirtfronts, she says. When one of the Porters throws a ball for the dogs onto Apple’s grass, security pounces, and a guard once advised them that “there are dog parks in Sunnyvale,” she says.

Between Apple and the Porters’ Bartlett Avenue neighborhood, Apple has erected a 12-foot sheet metal fence where an ivy-covered chain-link fence once ran. The new fence closely resembles another that’s gone up at Apple’s under-construction “spaceship” campus in Cupertino.

A couple of clues suggest that the shiny young techies who can be seen entering and leaving the buildings are working on projects connected to cars. City of Sunnyvale permit documents from 2015 describe an “auto work area” in one of the buildings in the complex. Another permit document, from 2014, refers to a “repair garage” in an adjacent structure.

Add together the mysterious noises, the permit revelations, and reports in this newspaper and elsewhere that Apple is working on some sort of car project — be it self-driving, electric or captivating software — and you have all you need for some amped-up speculation.

That mysterious hum that neighbors said originated in the complex could come from charging a high-intensity electrical capacitor, a piece of equipment “that lets you store electricity so that you can release huge amounts of electricity in a very quick period of time,” said senior MIT system dynamics lecturer John Carrier. “I wonder if they’re trying to build up a lot of electricity to see if they could surge-charge a battery. Could I charge a car up in seconds?”

If Apple is pulling a lot of power off the grid, night-time work could have an alternative explanation to secrecy: Off-peak electricity is usually cheaper, and massive power consumption during peak hours could cause a localized brown-out, Courier said. Most businesses in Sunnyvale use power plans with cheaper off-peak electricity, a PG&E spokesperson said.

Apple is neither confirming nor denying that the complex houses anything to do with an electric or self-driving car. “Yeah, no, we’re not going to provide an official response,” a spokesperson said. But the spokesperson, when asked about the information in the permit documents, and the strange noises, pointed out that Apple has a long history of developing auto-related products and services for gas-powered cars driven by humans.

What, exactly, are meant by “auto work area” and “repair garage” in the permit documents is unclear, as the terms are not defined in Sunnyvale’s building regulations, and the choice of words could have been made either by the service provider doing the work before Apple moved in, or the city staffer who processed the permit, said city of Sunnyvale spokeswoman Jennifer Garnett.

David Whitney, of David Whitney Architecture in San Jose, which applied for the “auto work area” permit, said he was not in a position to comment. Devcon Construction in Milpitas, which applied for the “repair garage” permit, did not immediately respond to a request for information about the permit.

But one thing is clear.

“Something is going on in there,” MIT’s Carrier said. “I can’t tell you exactly what.”

Contact Ethan Baron at 408-920-5011; follow him at Twitter.com/ethanbaron.