Sure it could be a sticky lifter or one that isn't pumping up but lots beyond the lifter could cause the sound: worn valve tips from improperly heat-treated stems, an adjuster that has backed out, broken damper or outer valve spring. You need to pop that cover and start checking things out.
It's helpful to isolate the sound with a cardboard tube held to the ear and valve cover. Then, using an old tin valve cover with the top horizontally sliced off, one can push down on the pushrod side of the rocker with a gloved hand while the engine is idling and often find the suspect lifter. That's the one that will be fairly easy to push down the p-rod side and clatter like hell...unlike the others that will show lots of resistance. Works for me!