Chevrolet Repair: loud tapping noise from engine, liquified petroleum gas, v8 chevy

Question
My mate has a loud tapping noise coming from his 5.7 litre V8 (Chevy Suburban 1500). Mechanics have replaced the big end bearings and now claim the problem to be "piston slap". To my ear it sounds to be tappets. As i have no experience with these engines I'm reluctant to start dismantling. He runs it mainly on LPG (Liquified petroleum gas). I've suggested that the valves and lifters should be checked. I assume that this engine has hydraulic lifters as the sound disappears except when at idle. As he doesn't have an engine manual I don't even know what gaps are required for valve clearance or where to set TDC. All suggestions/help gratefully received.

Answer
Hi Bern... you don't say what year motor is... you could be right as it could be a collapsed lifter. a compression test would help figgure out which cyl it is...you can find specs for this engine on the net...valve clearence is set by removing valve covers, back off rocker nut till rocker clacks, tighten nut till it stops clacking then turn a half turn slowly....hope this helps