Well I've figured out the "clicking" thing... and the Polaris isn't broken!

AFAIK the MIDI spec calls for the input on a device to be nothing but an optoisolator conected to pins 4 and 5 of the DIN socket... no other connections.  But some newer/cheaper MIDI interfaces, including the one I was using, "cheat" and steal power from the MIDI input by grabbing +5 volts off pin 4 and using the shell as negative.

Well in the Polaris, the MIDI output connector shell is connected to the chassis, and only to the analog/digital grounds through a 1-megohm resistor.  So when current flows through the shell, it energizes the chassis relative to the digital + analog grounds.  The current flow varies depending on whether data is or is not being sent out the MIDI port and I think that was causing the clicking I heard.  Just for reference, with nothing plugged into the MIDI port I measured a few hundredths of a volt between chassis and analog ground (at the PS board), with the interface plugged into the port I measured almost a volt!

It seems like what needs to happen is (other than manufacturers making MIDI interfaces that follow the spec, and good luck with that) to connect the MIDI connector shell to digital ground.  But doing that with the Polaris in the factory configuration would result in digital ground being connected to the chassis which I think would be bad for a number of reasons.  The only solution to ∗that∗ problem I can think of is to replace the MIDI output connector with an isolated one and I'm too lazy to do that.  ;^)