Macbook 13 inch - Screen Noise on attached display

I have a 13 Inch Macbook (2014) running Yosemite - the computer has been attached to an external dell monitor via the HDMI port. The screen works fine but on waking the computer from sleep, I see noise (http://ift.tt/1CVX1xB) on the external monitor. I have to plug out the monitor and plug it in again to make it work.