Engine noise/tapping; one for the books!

My 2009 Sonata 2.4 liter 4 cylinder is making a rapping noise that I first thought was lifters. The good news is there are no dash lights on and performance and economy are perfect. Odometer reads 30 K miles(!). More good, but mysterious, news; Starting and idling
and full throttle, even in quiet fourth gear, produce no abnormal noise. The rapid tapping noise occurs only at light throttle, but not below about 1800 rpm.

It's best demonstrated 'at the curb' with the lights and AC on to lightly load the engine at, say, 2000 rpm. It is only throttle dependent and occurs only at light throttle.

Using a contact stethescope and an ear horn is not very telling, but the noise source seems not to be under the cam cover, is definitely from the grille side, not exhaust side of engine. Is definitely closer to the belt end of the engine, but the chain cover is pretty quiet. It almost seems the Intake manifold runner control vacuum actuator is involved.

At one time the oil control valves (for cam phasing) were suspect, as they are throttle related. But disconnecting their plugs made no
change.

When I say no noise at full throttle, thats loaded, in any gear.

Go figure.