I would think it is a lifter leaking down at the given rpm. Below and above 1500 rpm the oil flow to the lifter exceeds the leak down rate but at around 1500 rpm the leak down rate exceeds the oil flow to that lifter causing extremely excessive valve to cam clearance. A friend's Altima 3.5 SE sounded like that at a given rpm but sounded normal either above or below that narrow rpm band. Turned out the engine was low on oil. I couldn't believe the valve train on the Altima could be that noisy. Mark