If this is the first time you installed the timing belt then the
problem is most likely you used the wrong marks.  Everyone makes this
mistake the first time.

Thanks for the suggestions!

With the help of a local excellent Subaru mechanic I found that I was
using the wrong mark on the crank sprocket.  I mistakenly used the
arrow at the front of the sprocket with the woodruff key to the right,
instead of the marked tooth at the rear with the woodruff key at the
bottom.  Fortunately, there was no apparent damage to the valves.  It
tapped for a minute or two after starting initially, but I haven’t
heard knocking since

Hey Nathan,

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