'94 Crown Vic Making Loud Noise

My 94 is making a loud noise,when I start it up is usually the loudest but some times all I got to do is put it into drive and it really calms down and all you here is a thumping noise,but it can get real loud again,I think its a camshaft does any one else have this problem and can help me??? Its a crown Vic police inter... :banghead: