clickity clack, my engine goes smack

[kjc292]

hey new to the site. I've always wanted to own an older chevy truck and a few months ago I finally made the purchase. I bought a 1978 c20 with a 292 and 3speed granny low. I almost passed on it but the 292 sold me. When i bought the truck it ran fine, no noise really, just a bad throwout bearing. well within the past two weeks a very pronounced clacking noise has came from what i believe to the upper block. when its in low idle its pretty much not there, but when accelerating and cruising(with foot on gas) the sound is there. now I've already tryed to tighten the lifters but no luck. I'm thinking a bent lifter. Not really leaning towards a bad piston or anything in the lower block just because its not a low knock. any ideas or what I should do?