Sitting here working on my laptop a while ago, I noticed a 60-cycle hum. I remembered having this problem a few weeks ago, and believed it was my H&K Red Box cab simulator. IIRC, it caused hum through my 2i2 when the phantom power was switched off. So I turned the phantom power on. Still, the hum. Or, maybe it was the ground lift switch? Ground, lift... still the hum, either way. Turn down the output level control on the 2i2. Still the hum. So maybe it's coming from the old Yamaha EM100ii powered mixer than I use to power my passive monitors? ****, maybe the 45-year-old filter caps are finally going bad. Switch the EM100's power off... still the hum. Turned out my kid left the bathroom light & fan on, then closed the door when he left. Fan off, problem solved.