GREEN BAY, Wis. —  Bob and Leona Ehrfurth say the noise that's been plaguing them for two years sounds something like a rumbling motor, with a subtle vibration that won't quit. Then it stops — especially when they try to show city officials or acoustic experts what they're hearing.

It's enough to keep 76-year-old Leona from sleeping.

"It's like there's a semi parked right outside with the engine running, but when you look out, there isn't one," she said.

She and her husband, who is 75, have lived in the same house for 42 years. The problem only developed over the last two years.

Her husband can sleep through it but also finds it irritating.

"It doesn't matter if the windows are open or closed — you still hear it," he said. "It's worse in the winter."

When they leave, the don't hear the noise, he said, so they know it's not some health problem the two share.

City officials hired a company for $1,000 worth of testing in the house this spring, but the tester came up with no noise and no significant vibration.

Alderman Andy Nicholson knows exactly what's bugging the Ehrfurths.

"Yeah, I've experienced it," Nicholson said. "It's like an engine thing, a low-frequency vibration. I think it would be an annoyance."

He had hoped the testing equipment could be used inside one of the factories in the area, but Municipal Judge Jerry Hanson wouldn't sign the inspection warrant because there was no reasonable suspicion of a violation.

The Ehrfurths' immediate neighbors haven't complained, although some people, like Nicholson, have said they heard the sound.

The couple said the noise started soon after St. Bernard's Parish across the street had the roofing, chimney and ductwork on a wing of its school redone. However, when the parish staff turned off all of its equipment as a test, the noise continued.

The city's Protection & Welfare Committee planned to take up the matter Wednesday night, when it discusses a report from Predictive Technologies Inc., which did vibration testing at the home.

Leona Ehrfurth said she's had to go to the basement or try to sleep in the sunroom to escape the noise.

"I try to stay in bed, but I get such a bad headache, I can't take it," she said. "We could move, but why should we have to? We didn't cause it."