Diagnose engine noise 4.7 L

Engine has "tapping" noise. Seems to come from everywhere when listening
by ear, is loudest in the valve covers and pan when using a stethoscope.
Does not show up at all in the timing chain/front cover using a 'scope. I
have replaced the lash adjusters because I was convinced that this was the
issue, but no change. In fact the adjusters all looked OK but I replaced them
anyway since I had the thing apart. The oil pressure holds at about 40 psi
most of the time but has a random low-pressure problem that doesn't seem
to have any influence on the tapping noise. Doesn't change with temp.
Doesn't change with engine speed except to get a little louder. Does not
have the characteristic hard knock under load or sudden acceleration that
rods or mains have. With the car in gear, brakes set, putting the engine
under load, the noise does not change. The engine is clean inside, it has
200,000 miles on it, runs just fine, but has the tapping noise. Compression
is good on all cylinders. Shorting out cylinders one by one doesn't seem to
eliminate the noise. Is there something flying around in these engines that I
don't know about? Thanks for any help you can give me.