I’ve had a similar/the same issue myself. Regardless of the power supply, I get the same noise described by others, which can be reduced using the depth control. I found that adding a decoupling capacitor to the ATmega328p fixed the issue. I soldered a 1uf 50V electrolytic across pins 7 and 8 (to the socket on the backside of the board, not to the IC itself!).
I think that the microcontroller was dumping noise to the shared ground plane. I may be wrong, but this has fixed the issue for me. I’ll let Brach weigh in on this, as mine may be an older unit.