When my cat, Hobbes, woke me up at 6:30am by licking the edge of a plastic garbage bag...I became borderline murderous. It's a habit that she's had since she was a kitten, and I find it to be the most irritating sound on the planet.
But, I suppressed the urge to kill, fed her instead, and then she does this. While purring. LOUDLY.
Ah yes, this is why we put up with our pets' crazy.