Sharp transistor cut off outside active region
I've found the source for the squeaking noises. It was caused by power supply noise propagating through the voltage divider and making the mosfet 'flicker' when approaching it's cut off voltage.
Adding a capacitor to the gate pin of the transistor solved the issue. I hade a 10u cap laying around and that's what I used. Just leaving it here in case somebody else decides to build this.