? About Ticking/Tapping Sound,

hello, i need some help trying to figure out why my car makes a ticking noise after it warms up. heres some background
91 4cyl lx 5spd with 120k. i just put this car back in action after sitting for 2+years, when i drive the car at about 2500 rpm(+/-) it starts to make a slight ticking/tapping noise(not detenation) it doesnt seem to affect performance but i gave it an oil change cause noone likes to use 2year old oil. the oil i used was full-sythetic 5w-30 and at first it seemed all good, but a few days later the noise returned. is it possible the noise could be caused by me useing thinner than rec. oil,use of synthetic or from sludge working its way out? i dont feel theres anything mechanically wrong (checked waterpump, alt and pullies for rubbing/skiping, havent pulled valve cover yet). any ideas or suggestions, i just need some input before i pull the valve cover and look for broken stuff. thanks

sorry about the double post

also, the noise seems to get louder and more consistant as the car is driven on short distances while on the highway the noise seems to go away or only slight happen.is there a "reliable" to check to see if low oil pressure is to blame for this 'cause i read somewhere that the oil pressure isnt supposed to change very much or at all on these cars, and since i changed the oil the oil pressure guage seems to move slightly up and down(just a hair though)
also forgot to mention that i did not use anything to flush out the old oil.