This sounds similar to a problem I had with my RSS feed
just as @s_ha_dum suggests it could be theme or plugin related:
a) is it a theme related problem? --> switch to the default theme 2012 to check.
if yes: You should also check if you have empty lines in between your code in functions.php:
or if there are some direct echos. In my case it was an empty line at the bottom of functions.php after
?>, so I now don't use
<?php at the end of the functions.php.
b) is it a plugin related problem? --> deactivate your plugins to check since some plugins can output empty lines.
if yes: then activate your plugins, one by one to find the problematic plugin.
To debug my RSS feed I like to use
and remember to remove feed caching if you have one.