Earthing. Make sure you have a good earth from your stereo to the car body and a good eart from your car body to chassis and from chassis to battery and from battery to the engine.
Make sure your alternator has a Suppressor like this
https://www.jaycar.com.au/car-radio-suppressor/p/AA3060
and make sure none of your radio wiring behind the radio is tangled together your 12v batt and 12v ign supply should be taped together and away from your earth and speaker wires as the problem your most likely having is electrical interference from other power wires or circuits in the car its hard to say what's causing it but these simple steps and checks should help.