Does it do it when it first starts up?
or only when hot?
which brand of lifter are you using?
are all the valves doing it?
After its hot remove a valve cover and find a valve that is in the fully seated position so there is little to no pressure on the push rod. Use your thumb and attempt to push the rod into the lifter by pressing on the top of the rocker arm just above the pushrod. It should feel rock hard if you feel any squish its probably a bad lifter. If you dont feel any squish is there any noticeable up and down movement in the pushrod it too should not move up and down at all
Masters of Industrial Technology intelligence thats why I own guns made to drop elephants.