It will be the Hdmi cable, have a check see if it is fitted right from the card to the monitor.
If is not that it is likely that it is a cheap Hdmi cable and is not have enough sheild to cut out any interference the power supply the monitor is out putting. another thing that can cause it is having desktop speakers with and amplifier too close to the monitor or hdmi cable. If none of these work for you try another cable of better quality.