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A camshaft lobe worn down or a lifter that has failed, so a valve no longer opens as far or as long as it should. On engines with switching rocker arms or cylinder deactivation this is a specific and well-known failure, because those lifters carry a locking pin mechanism that is more complex and more heavily loaded than a plain one.

It presents as a performance fault rather than a mechanical noise, at least at first. A rounded lobe reduces lift progressively, so the cylinder breathes badly, contributes less, and the module notices through misfire counts or rocker arm performance monitoring before anybody hears anything. A ticking noise arrives later, once clearance has opened up enough.

Oil is nearly always implicated, which is the useful part. These mechanisms depend on clean oil at the right pressure through small passages, so extended intervals, the wrong viscosity or a blocked feed screen wear the lobe that the failing lifter is riding on. Inspect the oil and its service history first — a new lifter into dirty oil fails again, and the camshaft it wore against is the more expensive half of the repair.