I whoop.
No hands free - I don’t do anything (except look impressed if that is appropriate under the circumstances).
One hand free - smack something, usually my thigh, maybe something inanimate if it is handy.
Even with both hands free I tend to hold my left hand and arm still and hit my left hand with my right hand, so it is a variant of one hand free (it dates back to badly breaking my left wrist many years ago, so it is not as flexible as my right one).