MILWAUKEE — A series of mysterious underground booms have been rattling an eastern Wisconsin city for at least two nights, baffling residents and local officials who have ruled out theories from earthquakes to water pressure problems.

The noises — described as rumbles of thunder, sonic booms or fireworks — were reported in northeast Clintonville, about 140 miles northwest of Milwaukee, starting Sunday night. The booms had quieted down at daybreak but started back up Monday night farther south and west, and continued until about 5 a.m. today, said City Administrator Lisa Kuss.

“There’s no warning, it’s just ‘bam,'” Kuss said. “I would describe it as startling, an adrenalin rush. … Your heart is instantaneously kind of racing because you are not expecting it.”

Local resident Al Miller said he’s been hearing rumblings for a couple weeks but chalked it up to thunder or didn’t think much of it. But an especially loud boom woke him around 3 a.m. Monday.

“My house shook and it was just like a shock,” the 71-year-old said. “I got out of bed and was like ‘Wow.’ I thought one of my trees fell onto the house.”

When he went outside and saw the trees still standing, he also noticed his neighbors’ lights popping on because they’d heard it, too.

Harold Tobin, a University of Wisconsin-Madison seismologist, said there are similar reports of booms in different parts of the U.S. and world from time to time. Sometimes they’re explained, sometimes they’re not, he said.

“I’m as intrigued and as puzzled as other people are,” he said today.

A seismic station near Clintonville has recorded unusual ground shaking since Sunday night. Tobin said such activity can be caused by quarrying, mining and heavy truck traffic, but since the city ruled out those sources — there are no mines or major construction in the area — the university and the Wisconsin Geological and Natural History Survey will likely take a closer look at the station’s data.

Kuss speculated that something within Clintonville may be causing the noises, since no one from outside the city has reported hearing them. No one has been hurt, and no damage has been found.

Authorities trying to track down the source found no gas in the sewers or problems with the city water pressure or wells, and nothing was found at an area landfill that might explain the rumbling, Kuss said. Engineers also checked a nearby dam and ruled it out.

There also were no recorded earthquakes in the area.

“It’s clearly not obvious to anyone involved what is going on,” Kuss said, adding that city officials were encouraging any experts who may have a theory to contact them.

Some residents also were frustrated and a bit sleep-deprived after being woken up for two straight nights.

“When it first started happening it was kind of scary,” said 21-year-old Jordan Pfeiler. “Then it was cool in a way. Now it’s just annoying. I want it to go away.”