Had the same problem with my GMT MASTER II BLNR which I bough from Harrods in Apr 2016 , I realised there was a small particle inside the dial next to the X "Rolex" took it in on the 1st of June to the Rolex Sevice Centre in London (ST James's Square ) they promsied to have it done before the 7th , a week as I need to travel. I'm eager to find out the reason how the particle has made its way inside the dial. I've vlogged my experience on my YouTube channel if you guys are interested:
https://youtu.be/qulPRZehf8o