SUNNYVALE (KPIX 5) — Residents in a Sunnyvale neighborhood say a secretive Apple facility is changing the face of their neighborhood, for the worse.

“At three in the morning, they have deliveries. It’s very dark, very secretive. We don’t know what’s going on, but almost every night there is noise that wakes the dogs up,” Joann Porter said.

Neighbors on Bartlett Avenue near the facility said security guards tail them when they walk their dogs. “You have security guards following you in their cars,” Porter said.

• READ MORE: iPhone Warning: Don’t Set Your Phone Back To 1970

But, it’s the noise that really has neighbors speaking up.

“You’ll hear like sheets of metal slamming, clanking, almost a grinding sound,” Jim Porter said.

A blurb in the June 2014 Sunnyvale City Manager’s blog offers a clue to the facility’s purpose. It says Apple was granted a building permit for “an auto testing facility.”

• READ MORE: Google Stands With Apple’s Decision To Fight Hacking Order

Neighbors said one sound in particular has them wondering, a sound like motors being tested. “It’s a high-pitched hum. And sometimes it can go all day,” Jim Porter said.

Apple has not responded to questions about what’s going on inside the facility.