Bah! bit of a nightmare project.

I think the noise stopped when I was using a capacitor is because I was holding
it with my fingers.

I have tried a transistor OR gate to ground the input pin of the amplifier.
The amplifier is connected to an FM radio circuit, and a pic pin that produces sound.
The OR gate can receive a busy signal from either the pic or radio circuits
to make it stop shorting the amplifier input to ground.