I think you still have the BOM on your site. Check all your custom PHP files like theme files, etc. Try installing Notepad++ and then run "Encoding > Convert to UTF-8 without BOM" in the menu, and save.
Or open the files, go to the very first character (Ctrl+Home) and make sure there's nothing before the <?php by pressing Delete. If you delete the < then the file is fine, but you may find hitting Delete the first time appears to do nothing/ Don't forget to put the < back if you do delete it.