DDRmedia 0 Newbie Poster

We have an oscommerce cart installed on a client's site. It uses authorize.net, and clients are seeing an error after submitting their order. The order however is going through. This is the error:
Warning: Cannot modify header information - headers already sent by (output started at /home/altovine/public_html/oscommerce/includes/modules/payment/authorizenet_aim.php:921) in /home/altovine/public_html/oscommerce/includes/functions/general.php on line 33

This is the code containing line 33 of /home/altovine/public_html/oscommerce/includes/functions/general.php on line 33:
}

header('Location: ' . $url);

tep_exit();
}

There is Nothing on line 921 of /home/altovine/public_html/oscommerce/includes/modules/payment/authorizenet_aim.php:921. The code ends at line 918.

Sure would appreciate any help with what this means and what to do about it.