According to Wikipedia, rats "emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, and when tickled."
Unfortunately humans can't hear the laughing without special equipment.
It's also been found they may have a sense of humor.
As part of their scientific paper, beguilingly titled Laughing Rats And The Evolutionary Antecedents Of Human Joy?, Jaak Penkepp and Jeff Burgdorf measured rats' responses to a hilarious series of anecdotes, jokes and limericks.
"We guess you just had to be there."Just don't get me started on why I googled "rat" in the first place.