Bought a 21.5" iMac on November 20th. I had to exchange 3 times because I kept getting screens that were half yellow. I was about to give up and get my money back when I got one that was "acceptable". the screen is not perfect but the best I have seen (including several display models from different stores) SO!!, I have been using it for a few days when I notice a knock coming from inside when the screen dims due to no activity for a min. while trying to investigate that issue, I am playing with screen brightness and notice a high pitched whine coming form it when it is between 20 & 80% bright. It is not loud but is ear piercing. The kind that makes you sick to your stomach after a few min because of the frequency. Thankfully the knock and whine do not occur at full brightness, which is where I keep it most of the time. BUT, I would like to have the option should the need arise. I now begin to research this issue and find that it is semi common. ARGGHHH!! My question is, Should i just return it and wait until they fix these issues or keep it? and if I do keep it, is it something apple can fix or is it just the way they are and a new module would do the same? I have purchased many apple notebooks over the last few years, but this is my first iMac and so far I am discouraged. Please give me your 2 cents.