Receiving error when accessing xml site map

Hello,

I have a multisite setup running several WPMU Plugins. I just recently began receiving this error when accessing my sites xml site map:

XML declaration allowed only at the start of the document

Here is the actual page: http://myfitbodybiz.com/sitemap_index.xml

It seems like this is often caused by extra white space in the php of either a theme or plugin. Is there a good way to try and track this down?

Thanks, Chris