Mysterious booms in Wisconsin

I've posted before about mysterious booming sounds from unidentified sources that have rattled people in California, New York, and elsewhere. Now, the small town of Clintonville, Wisconsin has been rocked the last few nights by weird booms. The city government has now hired an engineering firm to try to identify the cause of the noises. From the Associated Press:
If the firm finds the epicenter, it will try to pinpoint its depth and what is causing it. The city will spend $7,000 on the effort, but (city administrator Lisa) Kuss said "it's possible we'll never have a definitive answer."

The city set up audio and video recorders overnight Wednesday but didn’t capture anything. There was at least one loud boom at 5 a.m.

"Clintonville noises: City's equipment fails to tape mystery booms" (AP, thanks, Matt Drudge!)

"After 3 nights, sleepless Wisconsin town longs for relief from mysterious loud booming noises" (AP)