It's not a lifter. My former boss had a Jeep Cherokee that made the same noise. The dealer could never find the problem until a week before he traded it in. Though I do not recal exactly what it was, I seem to recall that it was an emission systems device for which there is a tech service bulletin. My hazy memory tells me it was the vapor return solenoid.

I'll call him in the morning to see what he remembers.

2500103 - DUTY CYCLE PURGE SOLENOID CAUSES A TICKING-LIKE SOUND FROM ENGINE COMPARTMENT
Date: 2/21/03

Model Year(s): 2002-2004

Description: This bulletin involves isolating the Duty Cycle Purge (DCP) solenoid from the vehicle body.

Details: The customer while sitting in the passenger compartment may notice a ticking-like sound when the engine is idling. The sound may be more noticeable in colder ambient temperatures. Mechanical contact between the DCP solenoid and the body of the vehicle may be the cause of the ticking-like sound.

The ticking-like sound may also be described as a tapping or clicking-like sound. The sound may appear to be similar to the sound produced by a worn or loose engine valve train component, such as a valve lifter.