1996 Ford Aerostar engine knocking when cold

This is a 3.0, it runs smooth but knocks badly when it is cold. I have changed the oil pump. And it still would knock the first time started each day for about a minute or two. The temp guage would fluxuate up and down when driving so I changed the thermostat. It now does not get to the halfway mark on the temp guage and continues to knock much longer. Then once it quits I try driving and after a couple blocks knocking starts back up. Seems that it quiets when at full temp and resumes when it drops back down. ( The knock sounds like valves, Lifters, or rocker arms. But I can hear it like it is in the exhaust. Could this be a injector or cadilatic converter problem? Please help. Thanks