Big Muff clone acting as tone generator

[Architect]

A friend of mine discovered yesterday that the Big Muff clone I had made for him acts as a tone generator when instrument cable is inserted improperly, only half way in. Playing with sustain and tone controls you can create sounds from cool melodic squeals to low crackling kind of stuff. I checked mine also and it does the same when the plug is half way in. Actually it's really cool, but can someone explain to me why that happens. My muffs are based on tonepad layouts. Also I have to note that tested stock pedal did not act this way.

[Architect]

Any ideas?

[Paul Perry (Frostwave)]

Quite often, if you have an amplifier and you disconnect the input from ground, you can find the gain suddenly going very high indeed (because, part of the gain determining feedback resistor network is ungrounded).
So, when the plug is only half in, you might find the gain is such that stray wiring capacitances cause the unit to howl at the moon.
If you traced out the circuit as it is when the jack is only part in, I think you would see what is happening.

[runmikeyrun]

wire in a toggle switch to switch modes!!