I built a RAT guitar pedal clone on stripboard based on someone elses design just because I had the 100% of the parts laying around already. The normal schematics don't include the DPDT bypass switch with LED circuit. I built one using a jfet, it seemed like it worked, put everything in a box.
Then I catch it oscillating when I turn up the gain knob, it was even audible in bypass mode for some reason. And the LED would turn on from it.
I guess step one is to take it back out of the enclosure and start checking the ground paths by the gain section?