You may want to check the nuts, bolts and darkroom grounding. Perhaps even flip the mains plug around--- yes, there CAN be differences.
I don't have but may also suspect that your singing units are the ones with regulated power, ESTnnn in contrast to TRAnnn?
From experience the TRAnnn don't normally and there should be little reason for them to sing. My magnetic regulators (typ Sola), however, are hot and hum and that's par for the course--- if they don't hum something is wrong.