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 (see image below) on the external monitor. I have to plug out the monitor and plug it in again to make it work.
It looks like timing issue between monitor and your Macbook.

You might try different cable or just turning monitor off and on with a button (much better than wearing off HDMI socket btw)