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.
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ahr888
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