Several residents on the west side of the Fox Valley reported hearing a loud boom Thursday evening, and we may know what caused it.FOX 11 heard reports from Omro to Poy Sippi to Fremont - an area of about 80 square miles.Dee Dee Goode described what she heard and felt Thursday around 6 p.m.“Holy cow what was that," said Goode. "My heart just started beating fast.”Goode lives and works in West Bloomfield - north of Poy Sippi.“We were just sitting in the house getting ready to go out for the night and there was this big boom," she recalled. "The whole house rumbled. It was the weirdest thing."About five miles away, Fremont residents heard something, too.“Just one big boom," said Tom Danoski.“It wasn’t no little gun shot, neither," added Wayne Seegers.They were surprised to learn it was also reported 16 miles away.“Oh, it was a heck of a bang then," said Seegers.Area law enforcement agencies told FOX 11 they received at least five calls about an explosion. Their immediate investigations did not reveal anything.However, a police report said authorities in Waushara County saw a jet in the sky around the time of the noise reports.A spokesperson for the Wisconsin National Guard said there was a military aircraft in the area Thursday evening for a training mission. He said, while there was no explosion, there may have been a sonic boom caused by the plane.A retired UW-Green Bay geology and astronomy professor said that happens when an aircraft moves faster than the speed of sound."Military aircraft aren't supposed to create sonic booms, but accidents happen," said Steve Dutch.Goode isn't surprised by that explanation. She said she's heard similar sounds before.“I guess we just assumed it was jets going through the sound barrier," she said.Goode is also relieved that what sounded like a major explosion caused no damage.An Air National Guard training base is 70 miles to the west at Volk Field.Two years ago, several booms were heard in Clintonville, about 25 miles north of Fremont. However, it's believed those were the sounds of an earthquake.