You do a "Hhhhhh" when you see someone REALLYATTRACTIVE and you want to explain how hot they are to your friends but you don't know how, so when you do a "Hhhhh" they'll know.
You see a super hot picture of a BTS member, and you want to say how hot they are to your friends. *Shows picture to friend* "HHHHHh" That's all the context they need to know how hot they truly are in your eyes.