I had exactly the same problems (Buzzing Sound & Greylines). Sent it back to Acer, they replaced the LCD and Motherboard. Got it back last week ran 3dmark06 no buzzing sound to be heard. The LCD Screen looks a little better.
I have had my 8204 for a couple of months now. And I have kept the powercord in for days.
And just the other day I installed NHC on too my Lappie and I noticed it said that the wear level was at 10% is that bad ???