You are away for a while and you girlfriend calls you up in the middle of the night because she misses you a lot, you are not going to be gone for long but it still feels like forever.
So after talking for a little while she starts purring over the phone, and that is your kryptonite. It doesn't take you long to fall asleep after that, and as you sleep you feel like you can still hear it.
Probably because she fell asleep on the other end, purring all the while
Art by the wonderful https://twitter.com/Alexahuke