CHICAGO (CBS)—It sounds like an alarm clock that doesn’t stop.

That’s how people living and working in Chicago’s Lakeview neighborhood describe a mysterious hum in the air.

They’re asking, “What is it? And why can’t everyone hear it?”

Even though the noise is subtle, it’s been driving some people between Clark and Halsted streets crazy. They hear it in the morning, noon, and night, and they’ve been looking for an explanation.

The sounds of city life echo through Lakeview, but lately something sounds strange.

Lakeview resident Lindsey Hoffman couldn’t ignore the noise as she walked down a usually-quiet street with her son Joey on Thursday.

“I hear it right now,” she said.

Another Lakeview resident, Stephanie Boroski, described the noise as high-pitched.

“Like an alarm sound, like a pulse but not really,” Boroski said.

The sound can feel “maddening,” Boroski said.

The mysterious noise has been reverberating through Facebook message boards too.

But somehow not everyone can hear it. Five-year-old Joey Hoffman said he heard nothing unusual.

Andersonville resident Kate Korte said she couldn’t hear the strange sound. Korte, who works in Lakeview, speculated that the sound probably blends in with the usual city noise.

“Maybe I am just muted because I live in the area and hear sirens,” Korte said.

Audiologist Dr. Akbar Razvi said some people hear sounds that others can’t because everyone’s hearing sensitivity is different.

“People can have hearing loss at different frequencies and some people can of course have normal hearing sensitivity,” he said.

Dr. Razvi said learning what the sound really is requires detective work.

Boroski said she walked around a Lakeview parking lot to try to determine where the noise was coming from, but she couldn’t figure it out.

A man who lives in a high rise on the 800 block of Grace thinks he has the answer: a massive air circulation unit on the building’s third-floor garage.

Residents nearby say they’re convinced it’s to blame.

“The noise is definitely that machine–that’s the noise we have been hearing,” Stephanie Wide said.

Building management informed residents that they have scheduled a servicing for the fan that circulates air on all floors of the high rise.