First things first. Are you grounded?
Yesterday, out of a sudden, I heard the hum/noise while playing; silent, but surely existing. Unplugged USB from the PC, plugged another guitar, another cables - nothing changed. Unplugged all except cable from poweramp in. Touched it, touched (stomp-box) poweramp. Ground! The noise disappeared. Plugged the guitar directly into poweramp - correct operation, touching the strings, bridge, etc - cancells the hum. So the issue is in AX8 I thought, damn! Unplugged AX8's power cord, plugged it into another socket (to be honest the same the poweramp was plugged in - thing you usually don't do). Noise went away...
Who was the murderer? Still don't know, I guess it was the shitty power extension cord / board, or, which could be more unfortunate, the wall socket itself. Allways check those little buggers, as they can kill the fun and lead you to think there is something wrong with the unit, the settings, noise gates and so on - which is usually just a waste of time.