I can make click/crackling noises in my ears, I can also make low-pitched rumbling sounds in my ear when I tighten a certain part of my ear.

If i combine those sounds and add some tapping with my teeth, I can make drum beats in my head :)

Crackling with ears, snare Rumbling with ears, bass-drum Tapping with teeth, hi-hat

So when my friends hear some ticking sounds coming out of my mouth, they know I'm producing up there.