HYDE PARK — A mysterious humming noise has returned to Hyde Park, perplexing neighbors, but this time there are good clues to its source.

Neighborhood resident Dean Armstrong walked the streets with audio equipment installed on his iPad, and found the sound was the loudest behind the Poinsettia Apartments at 5528 S. Hyde Park Blvd.

Armstrong said a chimney on the building is "very strong suspect" as the cause of residents' concerns — but said he can't say for sure. Building maintenance said they were unable to confirm the chimney is the source.

Since late fall, residents in East Hyde Park have reported hearing a low humming noise that comes and goes. Neighbors said they have been able to rule out easy answers like trains on the nearby Metra tracks, garbage trucks or fans in some of the larger high-rises in the neighborhood.

With no obvious answers, speculation began to range into the bizarre.

Some wondered if it could be tied to activity in the now-ancient underground tunnels used by the mob to smuggle liquor during prohibition or the University of Chicago operating a secret underground laboratory.

While the noise seemed to have stopped, it was a temporary reprieve.

“It went away in January and now it's back,” said Bob Drinan, who lives in the Windermere Apartments at 1642 E. 56th St. “It started Feb. 28, and we have been hearing it every night since then.”

Armstrong, who heard the noise late at night in his home at 5636 S. Harper Ave., was not so sure that easy answers should be ruled out. On March 8, he trekked out into the neighborhood with an audio spectrum analyzer, wandering alleys and under viaducts.

“I’m very much a science-based person, so when people have mysterious things happen, I want to explain it,” Armstrong said.

Just a block from his home he found the distinct signature of the hum under the 57th Street Metra viaduct.

“There’s this very prominent tone hitting at 85 hertz,” Armstrong said.

According to measurements taken by Armstrong there is a secondary noise near 30 hertz that is just barely within the ear’s ability to hear.

If the noise were a clear tone, it would be about as deep as the lowest F on a piano at 87 hertz and an octave below the lowest B on a piano at 30 hertz.

But the humming sound being reported is not a clear tone, and neighbors describe it more as a deep whirring noise like an engine.

So Armstrong said continued to wander checking other suspected sources, like the fans on the high-rise at 1700 E. 56th St. and elsewhere. He said the noise would register louder and softer as he walked around, but never changed frequency like a bus, train or other moving object would. The source of the noise clearly wasn’t moving, Armstrong said.

He said that in the alley behind the Windermere he began to hear the noise himself.

“I went up the alley and it just got louder and louder,” Armstrong said.

The noise peaked behind the Poinsettia Apartments at 5528 S. Hyde Park Blvd., registering 85 hertz, where he said he could actually feel vibrations, possibly coming from a chimney.

“You could feel something vibrating if you touched the exhaust chimney and you could feel something vibrating the ground,” he said.

But Armstrong said the sound might not necessarily be coming from the chimney, and it might be vibrating in response to the sound bouncing around the alley.

“In no way would I conclude that chimney is the source, I suspect it, but I can’t say conclusively,” he said. “It just seemed to me to be a very strong suspect.”

Armstrong said he went back out on March 12 to confirm an epicenter for the sound was around the chimney and was able to hear and measure the noise again.

Contacted by DNAinfo Chicago, maintenance crews at the Poinsettia said they checked the chimney that day and found no malfunction and didn’t hear a humming noise.

Anderson said he has no plans to take action regarding what he found; he was just interested in solving the mystery of the source of the hum.

“I’m just putting information out there,” Armstrong said. “If people want to do something further, I’m not interested in that.”