BOss OS2 oscillating needs help

[fretzburner]

My OS2 produces high pitched noise when effect engaged and it is responsive to gain pot.gain zero normal sound and when slowly turned up noise starts with varying pitch along the entire pot range.I traced the noise and it starts right at the guitar input of the board.But when bypassed no noise at the input.already changed capacitors in the two gain stages of the circuit.Maybe somebody here is knowledgeable for this.Thanks