te falta el mysql_fetch_array()
Código PHP:
$connect=mysql_connect("localhost","root","");
$db=mysql_select_db("sistemanoticia2",$connect);
$result=mysql_query("select * from noticias where id_noticia='$id'",$connect);
$row = mysql_fetch_array($result);
echo substr($row['noticia'], 0, 100);