The text equivalent of a heart attack, someone struggling to remember something, or a stroke, accompanied by chest clutching, usually used when the person using the expression gets a surprising or shocking text message. The length of the HNNNNNG depends on the severity of the action for example, a "HNNG" would be less dramatic than a "HNNNNNNNNNG"
The sound you make when you see a REALLY attractive person in public and don't want to embarrass yourself by shouting and having a loud conversation with a neighbour about how attractive they are