Onyx high pitched noise

I’m using Onyx amp for a modern high gain guitar distortion so I use it with maximum gain with clipping type in ‘screamming’ (even I need a overdrive in front of it)

When gain is at 95 or more it starts to add a high pitched sound which at first I thought it was feedback as it is expected, but it sounded a little different. so I did a test to know if it’s something else. I plugged modduo to my computer (no AC plug, only battery) input and recorded a little.
I’ve found that the high pitched noise is still there! it’s not feedback…

this is the record with only Onyx with gain at 100 and then at 94 with no noise. the noise is worse with an overdrive…

is this a ground loop issue? how can I get rid it? I’m forced to put a noisegate with a too high threshold killing the sustain