Investigators are trying to get to the bottom of puzzling reports from residents in northeastern Pennsylvania, who say they heard loud booms and felt the ground shake in the middle of night.

Since April, people living miles apart in areas of neighboring Bucks and Lehigh counties have called police about hearing explosion-like sounds that apparently rattle their homes and shift the ground beneath them, according to ABC station WPVI in Philadelphia.

Richland Township Police Chief Richard Ficco said the reports typically come in overnight before dawn.

"The ground shifting, almost shaking," Ficco told WPVI in a recent interview, describing some of the calls his and other area departments have received in the last several weeks. "The buildings are shaking, ceiling tiles are moving and windows are rattling,"

"Definitely disconcerting," he added. "I would say unnerving to some people."

Ficco said two of his officers have heard the unidentified noise, and one of them also saw a flash of light.

"There was a flash of light and maybe several seconds before he heard the sound, and then the other officer who was further away heard the sound later than he did," the police chief told WPVI. "They both thought it was coming from different directions."

Some witnesses have described the noise as a loud thud, while others said it's more like an underground blast.

Milford Township resident Samantha Ritter said she thought she heard a firework go off one night. But when she looked outside, there weren't any in sight, she said.

"I'm hearing like a firework kind of...sound," Ritter told WPVI recently. "I looked out the window, thinking maybe [it was] neighbors setting off fireworks or something."

The Pennsylvania State Police, who are leading the investigation into the reports, did not immediately respond to ABC News' request for comment Thursday.

Spokespersons for the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said both agencies are assisting state police in the ongoing probe.