engine gurus --- I have a quick question

I have a 1984 cherokee with the v6. the engine has developed a loud nock when not under load while driving. it blows no smoke, uses very little oil, it idles smooth with no nocks. but under slight throttle it will nock very loud. but as soon as I give it a little throttle the nock will go away. it does not nock until the engine is fully warmed up. I plan on replacing this engine but its not in the cards this summer... just wondering if there is simple things to check.. not a daily driver

thanks for any help.

I had a stuck lifter that did something simular. marvels mystery oil fixed it right up. its cheap and worth a try.