So I finally had a chance to pop the valve covers this morning.
Passenger side cover came off with two flat washers, one spring washer and a wire clip sitting in it.
The rocker arm for the intake valve of cylinder 2 was just dangling on the rod, as these retaining washers and clip were no longer there.
Put things back together, checked valve clearances again for good measure, and then fired it up. Back to normal.
Whew!
I wonder why this clip popped off. All I can think of is I must have somehow partially dislodged it when I was setting the valves on #2, which did need to be adjusted.
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