<?php
$result = $modx->db->select("snippet", "modx_site_htmlsnippets", "id IN($banners)", "rand()", "1");
while( $chunk = $modx->db->getRow( $result ) ) {
echo $chunk['snippet'];
}
?>
found this "random chunk":
http://modxcms.com/forums/index.php/topic,44979.msg266716.html#msg266716