Hello,
Are you sure it's your motherboard? Use a paper tube and try to find EXACTLY where the sound is coming from.
It could be a few things:
Possibly coil whine from your GPU
A fan is out of place/hitting something.
Your PSU might even be making the noise.
Since only you have the hardware - please use the paper tube trick (either from a roll of toilet paper, or roll up a piece of regular paper and put it to your ear) and find exactly where the noise is coming from.