Top Half of Screen Dark

This has been happening for some time. I thought it was the inverter, but maybe not. When I first start up my iMac and I see the grey screen, for a split second the whole screen is evenly bright, but then the top half dims. I noticed this starting a while back when my screen would dim after inactivity and the top half would flicker and make a buzzing sound, but that issue went away. Now the top half is more dim than the lower half all the time, even as I adjust brightness - the top half dims along with the bottom half, but is always more dim than the bottom half. If I adjust brightness down from full I hear a very faint buzzing sound.

Is this a hardware issue?

This is a 2008 iMac 20"