Sorry to kill the mood here... but the first thing that popped into my head was - wet teeth scraping. (no! you're doing it wrong!)
Why was THAT the first thing to pop into your head? I did have that happen once in high school when we both were smiling as we started to kiss. If that hadn't happened to me, I wouldn't know what you are talking about. That is the most jarring sensation! worse to me than fingernails on a chalkboard!