A persistent, unusual noise has a band of homeowners from three towns angry at the Covanta trash-burning facility in Bristol and at city leaders who they claim aren’t doing enough.

“It’s like a nonstop pulsing vibration, it’s kind of torturous,” Theresa Murphy said Thursday. “It’s why I sold my house.”

A flurry of complaints hit the Bristol-Burlington Health District last fall from a couple of dozen Bristol residents — along with a smaller group of Plainville and Southington people — all reporting an annoying around-the-clock industrial noise.

Homeowners concluded the trouble was coming from Covanta’s enormous trash-to-energy plant on Enterprise Drive, where incinerators burn more than 700 tons of garbage every day.

Some bombarded social media with demands for action, and others contacted the health district and Bristol municipal leaders looking for someone to intervene with Covanta, part of a multinational with annual revenues of more than $2.2 billion.

“It’s a generator noise, it’s constant. It’s also like a low-flying plane,” Ellen Slipski of Southington told Bristol’s city council last month. “It wakes me up every night. It impacts your health, your quality of life, your sleep.”

Some residents have complained that Bristol officials are largely ignoring the trouble, but Bristol-Burlington Health District Director Marco Palmieri said the opposite is true.

“This has been my number one priority. We’ve hired a consultant in acoustic engineering, we’ve reached out to academia — experts at Brown University and UConn — and we’ve done a lot of research,” Palmieri said. “But this isn’t like a routine noise complaint. This is much more complex.”

Unlike an ordinary noise complaint, the sounds from Covanta aren’t excessively loud and don’t trigger any sanctions under state law that uses measurements in decibels.

“These are pulsations, not vibrations. A consultant said an analogy is like driving on the highway with the windows rolled up; if you open just one back window, it causes pulsations,” Palmieri said.

“And each person hears it differently. I’ve been to one apartment complex where someone was complaining, but when we asked other people in the building nobody else had heard it,” Palmieri said. “We think part of what’s happening is that the Covanta plant is in a valley, so the sound ricochets off the valley. That could be why some people in Plainville and Southington hear it.”

Covanta has been the subject of noise complaints on and off for two decades, and so far nobody is certain about why. Residents acknowledge that there doesn’t appear to be a pattern: One can find the sound profoundly intrusive and exasperating, while somebody next door barely notices.

In 2017 postings on city-data.com, residents reported that same thing.

“I recently moved to Bristol and immediately noticed a strange low-frequency hum that goes through the night. It is not very loud, but it constant and penetrates through walls due to its low-frequency nature. It bothers me a lot,” wrote one person. “It is more than 1 mile from my house, I can’t imagine how people who live closer to it, sleep at night. To my surprise I could not find any complaints or other concerns from other people on the internet.”

A poster replied: “Some nights it’s louder than others. I guess the sound hits the side of the house and makes it sound louder, so we had to move our bed and put it on the opposite side of the room to be able to sleep at night. I asked our neighbors and the previous owners and they had no idea what I was talking about … like you, I am shocked that nobody else notices it.”

Slipski said the noise troubled her as far back as 2013, but said a call to Covanta led to an equipment adjustment that seemed to work: The noise near her went quiet for years. But it resumed in 2021 and is still troubling, she said.

Judging by the volume of complaints, the disruption spread significantly four months ago.

“It started for me and for most of the people on Oct. 19. We suspect it’s because a lot of trees got cut down by Eversource, so a natural noise barrier disappeared,” Murphy said. “I was driving home and noticed a bunch of trees at Cross Street and Lake Avenue were cut down, and that evening I started hearing this noise. It was the first thing I heard waking up the next morning, and it was nonstop.”

Murphy contacted police, but was told that it falls below the legal limits for a noise complaint. She said nobody at city hall would return her messages, and that Councilman Susan Tyler was the only elected official to show any interest.

“I called public works, I called the health district, I called the mayor’s office, I talked with people on the council. I expressed in clear terms that this is impacting my mental health, causing stress and anxiety and sleeplessness,” she said. “There was no sense of urgency or compassion whatsoever.

“I sold my house in December because of this,” said Murphy, who now rents a home in Farmington. “I kept hearing ‘be patient.’ But if it’s been going on for years and they haven’t done anything yet, why should I think they would now?”

Mayor Jeff Caggiano said there’s no quick fix, but he’s talked with Covanta and is confident the company is fine tuning its equipment to resolve the trouble.

“It’s complicated, a unique situation. The city would get involved if it was high decibels and something we could track, but vibrations are very different. The level of oversight the city has is very limited,” said Caggiano, who praised the health district for extensive work to find a remedy. “Covanta has acknowledged they have issues they need to fix. They’re working with the health district.”

Palmieri said Covanta has retained its own consultant, who identified some mechanical adjustments and new equipment that could help. The company had hoped that work done this month would have sharply lessened the noise, but neighbors said it hasn’t.

“Covanta has made it clear they’ve spent a significant amount for an assessment and are following up on suggestions,” he said. “The experts I’ve talked with say this is going to be a long haul. There are so many variables involved, and this is about harmonic effects. This is a mid-level frequently noise. It’s not like turning down the volume or putting on a muffler.”

Neither state public health staffers nor the federal Environmental Protection Agency could offer guidance or solutions, Palmieri said.

In a memo to the city this winter, Covanta regional manager Mark van Weelden appeared to commit to corrective action. Monitoring for two weeks in December confirmed a problem, he wrote.

“This survey did detect an unusual noise phenomenon. It is believed the sound may be emanating from the facility’s two ID fans that exhaust into the stack. Covanta is moving forward to explore the potential causes of this phenomenon. Covanta has engaged acoustic engineers to help define the root cause(s) of this sound, and, if necessary, to explore abatement options,” he wrote.

But in an email to The Courant, Covanta was less specific and left open the question of what was causing the disturbance.

“We are treating this matter with utmost seriousness and are working closely with city officials and third-party experts to identify the noise source. Should it be linked to us, we will promptly address and rectify the issue,” the company wrote. “Our operations are conducted in compliance with our permits, and we remain committed to transparently addressing the concerns of our neighbors.”