Amp hum.

My system used to be dead quiet with no music playing. No hum whatsoever. Lately I’m getting hum I can hear across the room in the right channel. Unplugging peripherals like Sonos Connect and Mac Mini has no effect. Touching the amp or pre amp changes the volume of the hum. The amp is a Mardigal Proceed HPA2 and the pre amp is an Audio research LS-16 (tubes). Speakers are Aerial Acoustics 7Bs. High quality cables throughout including power cords. Decent power conditioner bar and somesort of power conditioner that plugs into the main outlet used for the system.

Yes it is an old system but with music playing there is no detectable change or loss in sound quality. Nothing has changed in the electrical system of the home. No recent changes to the hi fi system.

After playing music for a few minutes and then stopping the music the hum is gone and does not return until the system is turned back on at a later time.

Is this anything to be concerned about? What could have caused this issue? Bad tube? What are things I should look for or look into? Or should I ignore it since it goes away after a few minutes and has no effect on SQ? Any help much appreicated.