Thanks to Michael, Randall, and Tony for their responses.
Embarrassingly enough, the solution was not the (obvious?) one.
The throw-out bearing is fine. So's the transmission.
The "swish" or "pfffft" noise had nothing to do with depressing the clutch,
though it does happen when the clutch is depressed (!)---and don't we
always suspect the worst? What I missed was >something else< we do when
upshifting. . . and that is to lift up sharply on the gas pedal---an
automatic, simultaneous motion; and as I learned this afternoon, it takes
will-power to depress the clutch without letting off the gas.
What I found was a missing nylon bearing (or sleeve) that introduces the
gas pedal shaft into the engine compartment. The metal shaft was just
riding in the hole on the sheet metal. The nylon sleeve must have broken
out sometime this fall when the noise started. The shaft-to-metal contact
apparently transfers the sound of the linkages from the carburettor without
actually rattling, per se. The linkages to the carburettor vibrate just
enough to cause a swishing or "pfffftt" noise transferred into the cockpit
when the gas pedal is sharply lifted. And I apparently lift the gas pedal
a little differently when shifting from 1st to 2nd, or from 2nd to 3rd (and
as I said, the noise was inconsistent)---so Randall was right when he said
I must be shifting a little differently depending on the gear! But how
easy to forget that shifting and clutching also involves these snappy, fast
motions of the gas pedal. Now that I know what it is, I can sit in the
driveway with the engine off and duplicate the sound, more or less---but on
the road it was masked just enough to be difficult to diagnose.
"Finding web of spider does not prove which spider spun web." --Charlie