2000 LS1 in a WS6...Just has new cam/lifters/baskets/pushrods and BTR springs installed a few months ago. Was driving around the other day and got a loud tapping from under the driver side valve cover. Pulled the cover off and all of the rocker arms were snug except the one in the very back. I was able to wiggle it very easy, and the pushrod would spin by turning it with one finger. I pulled the pushrod out and didn't see any damage on either end...Re-torqued the rocker back down and still has slack in it..Thinking maybe a lifter has collapsed?

 

If you look at the pushrods beside the one that is short, do both pushrods look like there at the same height? If the top of the pushrod that is loose, is shorter than the rest, it may be a collapsed lifte Forgive me, as a suck at drawing

If there both the same height, it might be a rocker/ valve seating issue, like carbon around the valves that keep it from comming up all the way, so there's a gap between rocker and valve tip

 

Cant judge it that way as pushrods move up and down as lifters roll over cam lobes

 

Are you able to turn engine untill both valves are closed

 

By the way your describing it, it sounds collapsed