HaLo-
Yeah, the damage could be very extensive... talking new engine in extreme cases <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/butbut.gif" border="0" valign="absmiddle" alt=':but:'> Does the noise go away as soon as the car starts rolling? For instance, can you roll at just a couple miles per hour and put it in neutral, and the noise is gone? If it just goes away when you drive, maybe you should look at your oil pressure. It may just be a little low on idle and not tension the chain enough to keep it from hitting the guide. If it goes away at a very slow roll in neutral, though, I doubt that's the problem.
Nathan