Here's my experience. Mine is 50 inch Viera plasma TV that I bought in Feb. 2012.
The buzzing's been bothering me for about a year but I got lazy to fix the problem since it was in a room which I don't use often.
Anyway, I decided to call up the tech before the 2 year Costco warranty expires.
Here's what the tech advised me to do.
1) unplug the TV
2) unplug HDMI
3) press and hold power button for 1 minute then release
4) check to see the problem remains
When I did what he told me to do, the buzzing went away and for about 2 hours, it's fine.
He told me that the problem might be the built-up charge in the transformer. By doing the above, it releases the charge.
I will wait and see if this fixed the problem once and for all but I think it is a good start for those who are afraid to open up the tv.