My cat will come running whenever the door to somewhere he's not supposed to be is opened. That's his real favorite sound.

That cat knows what's most important in life - sleep and treats:-)

Also: can opener

This is almost the cat in my profile picture.

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For me it's the front door or closet door opening.

My cat is deaf now at age 20 so he doesn't hear the treat bag anymore, but his sense of smell is still very much intact. :D