I have an Astron RS-35M power supply I bought in 2020. It has developed a HUM over the last 8 or 10 months. Not very loud, but def audible, but only on TX. When I TX with more than 5 - 10 watts it hums, then when I unkey radio hum stops. There is no hum on my radio signal and voltage/Amperage is rock steady. I took cover off to check if maybe transformer has loose bolts. Everything tight. No symptoms other than annoying hum. I have a much older Astron RS-35A (no meters) and that power supply dead quiet. I asked around in my radio club for help and the consensus was "some power supplies hum" and there are no other symptoms. I looked inside and everything looks new. So not sure what could be causing the hum. At first I thought the cover vibrating. When I TX, I push down slightly on top of cover and hum muffles a small bit so something inside causing it to hum.