The zetec is known for having a loud valvetrain. If you really wanted to quiet it down then you could put tighter buckets(tappets) in. Ford put in different size buckets in all of the zetecs, I'm still perplexed as to why. But the tolerences that come from the factory are pretty loose. But you would much rather have a loose valve than a tight valve. When I changed my cams I also replaced the buckets and it quited down my valvetrain a lot.
