Issues with HUM on DIY audio signal switcher using a relay
You might have either 60Hz sinusoidal magnetic fields or electric fields intruding.

To reduce the magnetic field intrusion, add GND path immediately adjacent (or immediately under) the signal path. Or place a big piece of STEEL, that is grounded to your Ground wire, under the entire circuit.

To reduce the electric field intrusion, you need a big piece of metal that is grounded, under your signal path.

If Efield, simply wrapping your hands around the circuit will have an effect (good or bad).