Twice now I have experienced my Watch making a strange noise and vibration for no apparent reason. The noise is hard to explain - it's a bit like a clunky chime, nothing at all like the other sounds the Watch makes, and it makes a vibration at the same time. The vibration is not the normal 'tap-tap' haptic the Watch normally makes when receiving a notification etc, but feels more like the iPhone 6's vibrating motor when I set my phone to silent mode. I do not receive any type of notification or message on either my Watch or iPhone when this happens, and the Watch otherwise appears to act normally (it has not frozen etc). Has anybody else experienced this or have any idea what it could be?