The only thing I could come up with to explain this is the vent
was having some effect on the pressure differential between inside and
outside of the cockpit. Although I could be wrong on that. All I know
is sealing the canopy fixed the problem.

Sounds like your letting more air in than your allowing to exit. Many
ships have this problem and need a bigger air exit hole in the rear
which should be 1.5 X the entry vent size. One way to check this
situation is with your altimeter NOT hooked to the static system, the
altimeter reading will change slightly as the vent is opened and
closed.
I have been able to find the bad area on a noisy canopy by pulling
down on the frame in several locations while in flight. When back on
the ground check all around the canopy with a feeler guage (strip of
paper works) the feeler guage will move freely in the bad area.
Hope this helps,
JJ