I've got a persistent engine noise that I'm having trouble diagnosing:

-Once per engine cycle (every other rev)
-Present at idle and every engine speed. It speeds up with the engine but does not change in basic character regardless of speed
-Does not change with engine load
-Shorting plug leads did not affect it

It sounds like a very loosely-adjusted valve, but I pulled both valve covers, checked the clearances, and found nothing amiss. While I had the valve covers off, I looked for signs that a rocker arm might be hitting a valve cover, and saw no evidence.

The ear pretty easily localizes it to the starboard cylinder bank; it can be heard most clearly through a stethoscope on the starboard valve cover and on the intake manifold.

Does anyone have any suggestions? I'm sorta stumped.