ok fellas first off check the lifters by pulling the cover on the passenger side of the motor. they are not hydraulic if they have anything that looks like a solid surface, if it looks like a dented surface with a "keeper" then they are hydraulic. now drain the oil and feel for metal shards as in worn metal pieces, now give it 4 quarts of deisel fuel in the oil pan and run it for about ten minutes this will act as a solvent and clean the oil channels out,next drain the deisel and replace with oil now run it again and listen for changes. if the clacking still continues change the lifters by pulling them out, put lithium grease on the new ones and slide them into place, next put the push rods back in their original holes and tighten the nut down untill all movement is removed when the lifter is in it's lowest position on the camshaft, give them 1/4-1/2 more turn on the nut and you should be good to go.......old school shade tree fix it by socket dropper
