We breed two kinds of Rats: Roof Rats and Norway Rats. Roof Rats are highly vocal and make sounds to express emotions like fear, anger, contentment and joy or amusement. When they are contented, curious or mentally engaged, they make a chittering noise. They mostly seem to do it around people, not other rats. Roof rats make various chirping noises at other rats for social, ratty reasons. Sometimes, if two Roof rats are annoyed with each other, they will tell each other off with an “eee eee eee” noise. If one is really fearful or angry, it will hiss like an angry cat to warn the other to “back off.”
And, yes, when Roof Rats are especially happy and playful, they make a kind of giggling noise that I am certain is their equivalent of laughter! While recording Qiong playing with Kitty, I just happened to catch him clearing doing it at 5:08 in the video. I’ve linked directly to the spot below: turn your sound up so you can hear it better!
I know what you are thinking: “But what about Dumbo Rats? Do they laugh, too?” Yes, they can. Researchers found that Norway rats also laugh, but it’s unfortunately at a higher frequency than most people can hear. Here’s a video where they’ve recorded Norway rat laughter in ultrasonic frequencies: