I would check valve stem height 1st., then see if the lifter feels soft (spongy), you can feel that by pushing down on the rocker (lifter side), if its spongy, it just needs to pump up, i've had to replace them, even on a new builds, I don't use anything but comp magnum anti pump-ups on all my BBs anymore, had too many issues with the basic lifters rattling/callapseing, even with adj. rockers.