Eliminating ground loop in Bluetooth speaker circuit
Probably RFI rectification by the amplifier.

There are a few things you can do: 
1) Twist your wires into twisted pairs, this will lessen the effect of magnetic field loops.



2) Use inductive ferrites to increase the inductance of the cables, high frequency currents take the path of lowest impedance. Using a ferrite on the cable can block high frequency signals

3) Shield your wires or your project (this is more expensive) Shielding provides a faraday cage which could block most of the high frequency signal from getting close to your electronics.

4) Shunt the high frequency currents back to the source, use capacitors (RC filters) to shunt the high frequency currents back to the source before they reach sensitive parts of your circuit.