Strange engine noise & high idle 98 SL

My 98 SL with 77K miles (1.9 SOHC, manual tranny) developed a strange
engine noise shortly after the last oil change. The sound is most
noticeable during idle. From inside the cabin it sounds like
buzzing/groaning sound which seems to disappear or become unnoticeable
at 1500 rpm. The noise comes from under the valve cover and from the
engine bay it sounds like a several very fast tapping sounds. It is not
constant but starts once the engine warms up a little and even then it
fails every once in a while during rough idle.
Then the car began to start very roughly as if it were running on 3
cylinders but that goes away after 1 minute or so. Also, I was getting
an aweful lot of pinging. Not just the "normal" pinging during upshifts
but far more excessive and also when accelerating harder in 2,3,4,5th
gears.
The other day I replaced the spark plugs, wires, coils, and coolant
temp sensor. The dealer cleaned the throttle body 10k miles ago or so
and I use fuel additives/cleaners every once in while. The rough start
is gone, the pinging is gone. But now the idle is very high between
1500-2000 rpm for a long time after starting and even with the engine
warm. The strange tapping sound is still there. Could the timing have
anything to do with this? I know the PCM takes care of the timing but
could the timing be so outside of normal range that the PCM can't
compensate? Any ideas what's causing the sound and the high idle?
Thanks.