rdklinc: I may have a surprise for you. I too am presently suffering the same symptoms and did many things that you have done -including swapping the mother board. I listened to the 'electrical sound like it's shorting out' sound and noticed it emanating on the right side. After a week, it started again. It became frequent -again. I took it in and they held it close to a week. They gave up and said for me to call Apple and forcibly 'beg' for a new machine if need be -due to it started before its anniversary date and now has passed. I guess the only good thing is that it takes about 45 seconds to start-up and type in the password before I'm at the desktop: that's tolerable… just! I pondered. After holding the machine, I placed it down and "click"! That was strange. While restarting, I lifted just the right side and heard the same 'click' but it didn't shut off. I then shook it, then nothing. When sign-in screen came on, I gave it a little shake and 'click' it went off. Ah ha! Now I was able to make it shut off...
After you removed the screen, you should've had a photo of the guts sitting inside before removal.