Dogs Laugh

But probably not at you, an article on abc news talks about scientists exploring this vocalization, which is being called a dog "laugh".
"What we found is that it had a calming or soothing effect on the dogs," said Patricia Simonet, an animal behaviorist in Spokane who has studied everything from hamster culture to elephant self-recognition. "Now, we actually really weren't expecting that."

"The laughing sound that they make is something that was not even considered a vocalization until this study was done," Simonet said.

Click the link for the full article.