I would check preload and I would press straight down on each rocker arm (lifter end) with a wooden hammer handle while idling & see if any of em get quieter or change tone & those are the bad ones. I'd take em out & dissassemble and clean & reassemble (light amt of light oil on the plunger for reassembly) but dont fill with oil & keep all the innards of a particular lifter together & as you know return lifters to the same hole(s) then preoil with the hex tool (20 deg ATDC #6 compression for DR side/90 deg BTDC #1 compreesion for Pass side) IIRC.