A couple of responses come to mind, but I don't know if one counts -
#1 - Silence. Sometimes you get in a "zone", and there's no movement and no sound, and you think "this must not be very effective", then you look up and see a mouth open wide, eyes clenched shut and a face so awash with mirth it makes you burst out laughing yourself!
#2 - Some people have spots that no matter who, or what milieu, if they are tickled, these people are catapulted into "Boogie Nights". I know it's pure mischief, but when you inadvertently elicit that partickler "moan", it cracks me up.