I just bought a 1975 vintage Tele and a Fender Twin amp. Clean, they sound good. Distorted, terrible. I am using a Boss DS-1 pedal, and CAN get good distortion, but I get this terrible high pitched sound that stays constant when the distortion is on. It only goes away when I turn down the guitar volume knob. But when I do this, if I want to go back to clean, you can’t even hear it. Also, if I turn the amp volume up and guitar volume down, it does very little to compensate. If I turn the pedal volume up, the squeal comes back. As well, I can’t even use my lead pick-up (closest to the bridge) distorted because that squeal comes back. I can only use it clean.

Anyone know where the problem might be? Guitar? Amp? Both?
Ask any other questions that might help figure it out.

Thanks