I've discovered a hum coming from my diy 5F1 tweed champ when the cable is unplugged from my guitar (but still plugged in to the amp) and the volume is around 6 and higher. No effects, input 1 (high). It doesn't happen when plugged in to input 2 but I assume it's because input 2 has a lower signal.

Voltages are fine, tubes are new, and I've tried substituting other tubes but still the same hum.

Is this normal or perhaps unique only to single-ended amps?

I don't recall this ever happening before and maybe it's just an OCD thing because it sounds fine with the guitar plugged in. I don't usually leave my amp running hot without a guitar plugged in to it but the other day I got lazy and turned on the amp before getting to my guitar and plugging it in and as soon as that rectifier tube warmed up all I heard was "WOOOOOMMMMMMMMMMMM."

The hum increases when I increase the amp volume.

If a video helps I could try to upload one.

Thanks!

Edit: the amp is fairly new, give or take 50 hours of use, and I rarely play it with the volume under 7 because...Champ.