Hi,
> Try the guitar into the Zoom & then go into the FX Return jack.
This is something new to me, Ill try it, thanx!
I have put Zoom infront of the amp Guitar > zoom > amp - and is normal, how is should be.
Later I have connected Zoom back to Fx loop, selected noisy presets (now with the headphones on) and went into the setup menu of each added effect / amp.
With muting an effect the noise was gone.
So I suppose that what you say -the gain could be the problem, maybe with the heavy distortion, flanger ..
But I didnt have too much time now to go deeper in setup, so thats Todo
Cheers