It’s not just sniffing that gets to me, it’s pretty well any human-generated noise that annoys. Headphones, talking, any electronic device (although possibly not keyboards, which is handy as mine’s not exactly silent – hmm… double standards?)
Eating. Heaven help us, eating. Not only am I driven incandescent by people eating with their mouths open, but the act of taking a bit can infuriate somewhat. Crunching on a crisp with the mouth open, rather than sealing the lips over the crisp to reduce noise is one of this trivial yet mind-blowingly large things that can really drive me to distraction. Yes, really!