Purr of the cats is definitely #1, followed by kitty "murps" - those little half meows that always sound like a question.

Cameron's giggle. Especially the one he makes when its a "that is silly!" giggle (there's different giggles for different occasions ya know )

Cats thumping down the hallway after each other

The hum of the ceiling fan (or just about any white noise. I fall asleep in a second with the fan on)

Silence after a major snow storm and the only thing you hear is the sound of your boots crunching the snow.

Pouncer's snore. And the moise made by the flick of his ever-twitching tail.

On another note - the sound I hate: Hubby's text message alert tone. grrrr.... its drives me nuts!