If you have driven in the rain it may be rain water that is coming in through the leading edge of the underbelly cover, a leak in the front cap that is channeling it to the underbelly or through a wheelwell or slide seal. The tires will spray hundreds of gallons of water a minute.
2020 F28 RKS Titanium
2017 Creekside 23 RBS Sold
2016 F250 Super Crew XLT Overworked