How to filter out an iframe from feed

One potential option is to use preg_replace to do a regex match on your content and replace the iframe with empty space.

So this function:

function rss_noiframe($content) {
    $content = preg_replace( '/<iframe(.*)\/iframe>/is', '', $content );

    return $content;
}

add_filter('the_excerpt_rss', 'rss_noiframe');
add_filter('the_content_feed', 'rss_noiframe');

Should automatically convert any instances of <iframe src=...>...</iframe> to a blank.