Toilet noises
When I am in the toilet at my office, if I find that there's someone in a stall and I know that I will be gone before they come out, I love making strange and disgusting noises, 'cos they'll never know it was me.
I make really noisy nose snorts, farts or just general 'UUuuuurrggh' or 'Aaaaaaahhh' sounds whilst I am having a pee. It's hilarious!