Try a ground loop isolator.
Also, check your ground is good. Common error is the power ground being a worse ground than the RCAs' sheilding, hence that's used for ground which equals noise.
In addition, inverters tend to introduce a 50hz noise to the power (square sine wave for AC instead of analogue curve) which CAN be OK if you don't have ground problems or EFI issues.
First port of call is ground loop isolator.A multimeter is also invaluable to check the resitance to ground on all your paths to ground so you can make sure the power supply ground is the lowest resistance.