I took my cover off to inspect when it first started ratling and noticed that there is a little ridge of plactic that is supposed to keep the cover in place but i could see that it had worn down. When i reinstalled the cover ( making sure what was left of the ridge was i place) the tapping stopped for about a week then came back so i shimmed it and no more problems since.

1996 Zetec MTX NO Mods it's only a matter of money now though.