Are you sure?
If you dim the screen and listen above the imac. Do you hear a buzzing/short-circuit sound?? It can also happen if the cpu is above 50C.
This is bad.
No buzz on my machine at all (2.93GHz i7).
As far as I can tell, my machine is a showroom-quality machine. No buzz, no yellow tint, no dead pixels... and fast, fast, fast. 256GB SSD, 2TB HDD, 8GB RAM.
Just sheer iMac perfection.
It does get hot (as designed) when it's been Handbraking for 20 minutes straight, but if you think that's hot, you should have felt my 2006 Mac Pro doing the same thing. It would develop heat-related RAM ECC errors unless I ran the fans at a minimum of 1300 RPM at all times.