The Milwaukee Journal Sentinel said that Muskego police yesterday received reports of "loud booming noises" in the city but were unable to determine the source of the sounds.
Meteorologists think residents could have been hearing "frost quakes."
The sounds were reported just after noon and rattled homes' windows, according to a post on the Muskego Police Department's Facebook page.
"Investigating officers were unable to determine a source of the sounds or any evidence that they originated in Muskego," the department said in the post. "As of this time, we have not received any reports of damage or injuries from citizens."
The department noted that social media posts indicated the sounds were heard outside the city, too, including in Waukesha, Big Bend, Wind Lake and Vernon.
WISN-TV Chief Meteorologist Mark Baden tweeted that the likely culprit was frost quakes.
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