Matthew,
I don’t remember experiencing this particular wooshing sound before.
I know you have experience building circuits, but i have to say this…please make sure all the wires (xlr and piezo) are hooked up to the correct place (don’t forget to check in the xlr jack). Also check with your multimeter to make sure you are getting a very high resistance (almost infinite) between the piezo disc wires…I’ve built hundreds of these things and I still make wiring mistakes sometimes.
It sounds like you know what you are doing on the pcb side of things, which makes me think this issue might be related to the piezo disc. If you unsolder the piezo disc wires from the pcb and plug the XLR in to the preamp, do you still get that sound? If you don’t, i would suggest trying to hook up a different “test” piezo disc (without even shielding it) just to see if you get more expected results.
If you still get the noise when the disc is unsoldered then there’s a good chance that it’s a component failure. It’s pretty unlikely to have a component failure, but still possible.
Let me know what you find.
-Brach