Sizzling electrical noise

Hello,

I have a iMac 27" medio 2011 and since a few months I sometimes get a sizzling,electrical noise from inside the iMac.

The sound is related to the brightness of the screen as well,when I lower it to 60% the sound is gone,when put to 100% it makes a really loud noise.

When scrolling the sounds is worse as well.

Anyone has a solution for this problem?

Regards,

David