Error only on www. page

On my site: comes up with the error:

Warning: Cannot modify header information – headers already sent by (output started at /home2/paulmil/public_html/wp-blog-header.php:1) in /home2/paulmil/public_html/wp-includes/pluggable.php on line 1207

However, without the www

loads fine.

I think it is an error with a plugin or a loading script…I am at a loss at how to figure this out!