I did a bit more testing but came up inconclusive.
The resistance between GND and earth seems fine, a couple of ohms on my crappy meter that reads 1 ohm with the probes shorted. Something odd that I noticed with an analog ammeter is that the needle will drift cyclically up and down a few milliamps. As a point of comparison I checked three other ATX PSUs, all from different decades and none showed current from GND to earth. I did the same test to the one in my PC with everything disconnected and got no stray current, so this phenomenon only occurs with the whole system hooked up.
Another more concerning and puzzling thing that I found is that with the ammeter attached to GND in the PC, and earth, the needle jumps around in tune with a new squealing sound that the PSU is making. I don't know when this noise started, but I only noticed it when I went snooping around my computer. Remind me to never buy a corsair power supply again, because every one I have owned, new or otherwise, has squealed like a stuck pig.