I’ve got the same thing, but it still happens with the dew heaters or without. Very noticeable, and drives the cats insane. Same frequency as the loud beep when started up, but quieter and continuous, right?
I posted the same issue below and sent the log to zwo but no response (except acknowledgement) yet. Very annoying- you can’t have that sound in the back garden - it will drive neighbours pets insane too.