Samsung UE40ES7000U E-Manual page 777

If the humming sound is loud, you may
have set the brightness on the TV too
high. Try setting the brightness lower.
You can also have loud humming if the
back of your Plasma TV is too close to
a wall or other hard surface. Also, try
rerouting your connection cables.
An improperly installed wall mount can
also create excessive noise.
Image Retention (Burn In) Issue.