73 bus, engine noise, oil light

I hear a knocking sound like a rocker hitting
push rod or valve, so I checked and adjusted
valves. Still makes the noise. Valves are all
at .006

doesn't make the noise at low RPM, but as soon
as I put a load on the engine, the noise begins
to gain volume.

after checking valves, oil light has come on
a few times. Plenty of oil in the case.

when I changed the oil last week, there were
chunks of valve cover gasket in the oil
screen (any other engine gaskets made of cork?)

the bus is new to me, the valve cover gaskets
were in OK condition when I bought it. No chunks
out of them. It looks nice and clean under
the covers.... hmm. no oil dripped out when
I checked the valves. Is this normal on these
engines? I know the type II engines have oil
sitting in the cover when you remove it, and
it makes a mess.

Perhaps the previous owner had a valve cover gasket
blow, and chunks got in the engine? Blocking oil
passages in case, not allowing oil to flow to the
oil pressure sensor? This engine has an oil
filter.

if this is the case, how can I flush the blockage
in the oil passages in the case, and should I
troubleshoot the problem?

Jen