Ok, i do not suspect your index.php causing the problem. I could be wrong but I will know for sure when I get home later and am able to get on my dev computer to compare index files. All I have to work with right now it notepad
So, I think this is a addon or template issue.
Have you installed any additional addons?
*Something* is adding white space to the top of your html code. This is what's causing the feed error.
List any non packaged addons and check for white spaces at the beginning and end of every template file you use.