How to fix 0021 error code?

I've installed the iFixit Red Ring of Death fix kit, but I still got the 0021 secondary error code. Followed the instructions on iFixit and reflowed the motherboard with a heat gun twice, but with no success as well.

Here's the link to the description of the error code: http://www.xbox-experts.com/errorcode/E0...

Also, when I turn on my Xbox 360, there's a sound that comes from the disk drive. Kinda sounds like a drill from a dentist's office, but only lower pitched. The 3 Red Rings soon follow.

According to the link above, the problem may be because of a "...bridged solder joint under GPU or Southbridge...", "...power related...", or "...missing components. " This is my first time trying to fix an Xbox so I'm not sure what a "southbridge" is.

To my understanding the only way to fix a bridged solder joint is to reball it and I heard it's pretty costly. Other than that, how could I test if the error code is power related or due to missing components and if either one are the cause, how, if possible, would I fix it?

Also, if you suggest reflowing the board, what would be the next option besides using a heat gun (something that I could do at home perhaps)?

Thanks!