OCD problem

[Ecc83]

hello. i have a problem with ocd copy. every thing works ok, but when FX is in true bypass mode, something like an oscylations appears (fx is squealing. this "squealing" appears only when volume, tone and gain are set up from the half of the scale. it's strange, because the "squealing" shows only when FX is in true bypass. Do you have any idea what can cause this high tone noise???

[anchovie]

I had this too, even with the 3PDT set up to ground the input in bypass. Oscillation went away when I used shielded cable (with shield connected to ground) from the input jack to the footswitch and from the switch to FX-in on the board.