Cannot modify header (for RSS)

I’ve gotten this error:

Warning: Cannot modify header information – headers already sent by (output started at /home/dsphoto/public_html/index.php:2) in /home/dsphoto/public_html/blog/wp-includes/feed-rss2.php on line 8

I know that this has to do with whitespace and the php tags, but for the life of me, I can’t figure out where the phantom whitespace might be. Here’s the code for the index file (that I haven’t modified at all since installing):

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./blog/wp-blog-header.php');
?>

Any suggestions?