Worse than I thought.  Seemed drivable, so I headed to the dealer
(about 3 miles away).  Within 1/2 mile the minor chatter became a full
blown racket, power dropped to near zero, stalled, wouldn't restart,
and I coasted to a stop.  Was able to push to a side street with the
assistance of a couple of helpful strangers, but had to be towed
(flatbed) to dealer.  Waiting to hear the diagnosis.  If it was the
turbo, I hope it didn't send any fragments downstream. Can't figure
what else unless the valvetrain came apart.  No odd sounds from
alternator or fans, no electrical problem; couldn't find any vacuum or
exhaust leaks.  Fortunately, and this was the only time I ever did
this, I purchased a 6y/60k extended warranty from Subaru of America to
ensure the turbo was covered.  I've had the car for 4 years and 40k.
Have good maintenance records, although I didn't have the dealer do
30k service (have a trusted local shop that went through the
checklist, replaced the serpentine belt and radiator hoses-probably
could have covered under warranty, but wasn't expensive and these guys
treat me well).  Let's see what happens.