We just bought a 1994 Ford Bronco, my wife was.

We just bought a 1994 Ford Bronco, my wife was driving and heard water flowing on the passenger side. I went to drive the truck didn't here anything. About 10 min later my wife said do you here it. She was right; sounds like water flowing under the floor board or at the glove box, to the floor on the passenger side. Strange I found no moisture just under the trucks air system. Anybody ever heard of this and What is it? Please let me know what I might need to do about this noise and is it or will it be a problem. Preventative maintenance is key. Thank You