Basically,
guys that make noises like the cats they are.
An accepted stimulus to produce the
purring in the subject would be scratching under the chin (or the back of the neck), caressing his head, a gentle
backrub, or (not for the
weak-hearted) nipple play.
I personally know three guys who purr for none or all of the above reasons, myself included. Purring during foreplay is apparently quite arousing to the partner as well. However, abuse of purring is discouraged on the grounds of possible interpretation as total weirdness.