1995-2002 Range Rover P38A
Sounds very much like the valvetrain. It might be a sign of low oil pressure but more likely rockers. It sounds like you have more than one that has suffered the sinking socket problem. A 98 may well be in the VIN range that was affected. Pull the rocker covers, remove the rocker shafts and have a good look at the cup in the rockers where the pushrods sit. You are looking for ones that have sunk into the alloy rocker. There was a Technical Service bulletin issued that highlighted this problem.