changed spark plugs on my 94 lincoln town car and now i have tapping sound, what could it be?

You may have a cracked spark plug. The noise is usually the combustion mixture being pushed out through the plug, eventually it will detonate. It is hard to tell by looking at the plug if it is cracked, you may want to buy another plug and cycle it through until you find the cylinder with the bad one. You should be able to tell which side of the engine it is by listening to it.

or you dropped some junk in the plug hole and its in the top of the piston,or it could be carbon,a little water gently introduced into intake vacuum hose will remove carbon,just dont overdo it,you will be sorry.