98 Ranger 2.5 noise in motor /tap

hello,there I have a big problem and I cant find out what it is.My 98 2.5 99,000 miles stick has been sitting for about 7 months and I went to drive it and it started to tap under the valve cover.I called a friend that works a dearlership and he said they have roller rocker lifters and to change the oil and add resolve to the oil so I did that and the same tap noise.I checked the oil and no metel was in there.I drove it for about 10 miles and nothing changed.So I was woundering if anyone could please help me.