I've built a few guitar pedals using op-amps, transistors, etc. (with supporting resistors & capacitors of course), and I've generally 3D printed the housing for the components for custom designs. With these circuits, there is generally static and sometimes radio stations being picked up and amplified. On the pedals and circuits on my full metal test rig, however, I almost never have any interference and noise problems. My best guess is that the metal acts as a Faraday cage and blocks most, if not all, interference that the plastic boxes instead let through. I've thought of different ways to shield my 3D printed pedals with aluminum foil or other metal, but I'm sure there is a way I'm not thinking of. How can RLC audio circuits be shielded in a plastic 3D printed pedal box?
The legendary Marshall 100W Super Lead adapted for use as a distortion stompbox is one of the schematics (of many). Schematic from Runoffgroove.com.