I’m sorry for the delay in responding. I just got back in town.
This is a very strange issue. It almost sounds like the PWM from the microcontroller is making its way through the optocoupler to the signal path. My initial thoughts are to ensure that all the components are installed correctly (proper orientation) and to ensure all the solder joints around Q1, opto1, and the trim pot are solid and not shorted to anything. Also ensure that the solder joints don’t have too much solder on them; sometimes this can cause the joints to act as antennas, broadcasting the signal to other parts of the circuit. Also make sure all the components are installed correctly.
Are you testing this with the sine wave function? You should be for these tests.
You mentioned that you adjusted the high trim pot. Please move it in the opposite direction that caused the purring sound. It could be set too high. Ensure that it is working properly; you should be able to turn the high signal off by adjusting it all the way in one direction (25 turns). Does purring sound go away when you adjust it to where all the high signal is gone, or does the purring stay the same or get louder?
-Brach