Mine does the same thing when last or in middle of a pedal chain. I’ve had it apart and checked/tested everything. No amount of knob tweaking or pot adjustment made a difference so I put it first in the chain (sonically not the best for me) and LO, noise whine much less. Standalone it’s quiet as a church mouse. Input impedance mismatch? Hope Brach can come up with some ideas…
Scooter