Re: Underground "sonic booms" in Wisconsin
Cave Collapse, probably. This area of the state has lots of dolomite bedrock, and is known for karst formation. Here's a link: http://wisconsingeologicalsurvey.org/karst.htm. Follow the maps link to find a map of dolomite / carbonate bedrock in the state.
Something similar happened to a relative in Tennessee. Very loud booms. One look at an airphoto showed obvious signs of caves in the area, like sinkholes and depressions. Over time, a sinkhole has been growing near the neighborhood.
These things tend to happen more when the water table is down, as there is less hydrostatic support of the cave roof.