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…

https://clyp.it/hb1gqfyi?token=92757d2f5947cf722912ac38c673fe87
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