Anyone else has a Nexus 4 that randomly makes a strange notification-like sound? Mine did and after searching high and low I established it was my transit card that caused it. Every time the card came close to the rear of the phone it happened - obviously as the phone tries to communicate with the card via NFC. Turning the NFC function off (or ensuring the card doesn't get close to the phone) solved the problem. Do this by going to Settings>More and uncheck the NFC option. Obviously any other card/device that works with a proximity sensor (like an access card at work) would have a similar effect.