An audio thump at power up is almost always because the power amp is
being turned on before or same time as the source. Audio gear thatnot
pinball often has a delay circuit built in to alleviate this problem.
Also, on a pinball you have no control of this since there are no
separate circuits to power up and the power amps are "integrated" into
the boards.
I don't have a schematic for Corvette so I can't really comment
specifics, however if the noise does bother you there are plenty of
delay/relay circuits that can be built to insert a relay to the speaker
so that comes on a second or few seconds after everything else does.
It's up to you if you feel the time trouble and expense is worth it.
I'd probably just live with it IMO.
--
wayout440