The noise from the last project log was still present and it has been driving me crazy for the past few days. It took me lots of failed attempts to isolate the cause until I had my long-awaited eureka moment.
Turns out the wiring for the battery was touching the wiring for the electret microphone. And this was causing interference. I separated them and nade sure they are far away from each other and the noise finally went away. Hopefully this is the real reason. If this turns out to be a ruse I'll definitely be pissed.