So you are validating your WordPress blog feed and you get an invalid character error. The validator even shows you where the invalid character is located, but it looks fine in your browser!

Copy and paste the text into a text editor and you will see it. In Windows Notepad, you will see a small rectangle, and in Notepad++ you will see more – in my case, the letters “ETX” on a black background.

How did it get there? Probably because you copy and pasted your text from another source.

How do you get rid of it? There are a couple of ways:

One, find the offending post and rewrite the word before and the word the validator was pointing at and delete the original words to hopefully delete the invisible character. Then republish.

Two, create a filter  in your functions.php file to filter it out of existing posts.

function wpc_rssContent($content) {
     // Search for end-of-text character which is created by using      // chr(3) and replace with blank string.
     $content = str_replace(chr(3), '', $content);
     return $content;
}
// Filter the Excerpt RSS feed
add_filter('the_excerpt_rss', 'wpc_rssContent');
// Filter the Content RSS feed
add_filter('the_content_feed', 'wpc_rssContent');
/* Filter the Editor content to keep the character out of your 
   posts before you publish. You will need to click Save Draft at
   least once before publishing to ensure it is removed.
 */
add_filter('the_editor_content', 'wpc_rssContent');

There is probably a better solution out there, but this worked for me.