Keep in mind, when you find where to start looking, that this can be caused by something as simple as white-space. For example a space or return before the opening <?php tag in a file or after the ‘ ?>` tag at the end of a file or even the white-space generated by something like
<?php // a line of code ?>
<?php // another line of code ?>
This is why I say you need to narrow this down to what plugin or maybe the theme.
