tapping/chattering noise

car is a 1997 taurus gl wagon 3.0 V6 (non-OHC) trying to sell car and it started a loud tapping sound, it is not a consistent tap sometimes it will chatter and tap. when it stops the idle goes down a bit. the only recent problem I've had was a cam position sensor which I replaced along with the distributor thing that it sits on.The noise sounds like it is in the front valve cover. I was curious if it may be a lifter. the valve cover looks fairly easy to remove. but what should I look for and what will I be looking at when I remove it. It taps and chatters for 15 seconds stops for 10 seconds comes back for then taps for a while then chatters then stops again for a few seconds then repeats in a random fashion.