The sound that hyenas make sounds very similar to human laughing.

Listen here:
http://www.georgetown.edu/faculty/ballc/animals/hyena.html

There is a species of hyena, who are called laughing hyenas. But actually hyenas don't laugh. When this creature is on the prowl or becomes excited, it makes a howling and chuckling noise that sounds like a laugh.

when hyenas are being attacked they make a giggling sound i think this is where term "laughing hyenas" comes from