XML Feed invalid in drupal 7
I wrote a detailed description about this problem and a possible debugging solution here: https://drupal.org/node/1275042#comment-7814971

In my case, Views Slideshow was problematic, I just created a tiny patch for that:
https://drupal.org/node/2078689#comment-7815015



For convenience, I'm copying my post linked above:


  As merlinofchaos said, this really has nothing to do with Views. The
  "fix" in #6 is NOT actually a fix...
  
  In my case, I had two whitespaces in front of the &lt;?xml tag, so the
  RSS feed started like this: "  &lt;?xml". I never thought I will find the
  source of the problem with such an ugly solution, but this is quick
  and works - it's really dirty, so use it only on a local development
  copy, do NOT test it on a production site:
  https://drupal.org/node/175137#comment-1526410
  in includes/module.inc file, in the module_load_all() function, after the
  drupal_load('module', $module);, soyarma suggests you to put in the
  following line temporarily: print $module.'|'; This way you'll get a
  list of your modules in the beginning of the output separated by the
  pipe (|) character. When there are unnecessary whitespaces before the
  module's name and the pipe character, then you just found the buggy
  module.
  
  For example, in my case, Views Slideshow was the source of the problem:
  views_bulk_operations|views_content|  views_slideshow|views_slideshow_cycle (this is just the end of the huge list)
  
  As you can see, there are two spaces before views_slideshow. This is
  where I should start searching for the source of the problem. I found
  it in views_slideshow/theme/views_slideshow.theme.inc: the code starts
  with two whitespaces, and the PHP block is just after that: "&nbsp;&nbsp;&lt;?php"
  You can also see it here:
  http://drupalcode.org/project/views_slideshow.git/blob/d9fd60c20add43a7b...
  
  And YESSS, after deleting the two spaces, http://validator.w3.org/
  congratulated me that my RSS feed is valid. Thank you, soyarma!
  
  Don't forget to revert the changes in module.inc (don't ever hurt the
  core's code! :) ), and delete the print $module.'|'; line.
  
  I'll report an issue to the Views Slideshow's queue, and I hope that
  this helped someone.
  
  ====
  EDIT:
  I reported the bug to the issue queue, and also created a patch, see #2078689: Unnecessary two whitespaces before the PHP code
  block in views_slideshow.theme.inc, which can cause RSS feeds to get
  invalid.