Contented noises people sometimes make after a good hearty companionable laugh.
The noises themselves may be individual to a person but usually are in the form of an 'aaaahhhhh', 'mmmmmmm' or 'ooooohhhh'. The noises are akin to those one would make when settling in front of a blazing fire after a hard days work. They are probably a manifestation of the endorphin release with which one is rewarded through laughing.
If a group of people share a laugh, they will sometimes subside into producing these noises. They will smile beatifically at one another or stare vacantly into space. The noises can sometimes provoke a new laugh if one becomes conscious of them resulting in a pleasant self-sustaining cycle. One can while away many hours like this.
Post laugh noises may not be made following a gigglefit as the sufferer may be incapable of further vocalisation. Jokes which are cruel,sarcastic or overly sick may also not produce the required contented feeling.
I'm not aware of a word in the English language to describe 'post laugh noises'. A reputable person has informed me that the Japanese have a word for it.