you must be close to a radio station transmitter.
sometimes its a actual commercial station, maybe a church or even a home Realtor.
sounds like the amp does not have much for radio interference protection. simple circuit really , one resistor and one cap.
you could try and install one in your bass instead of the input jack on your amp. pretty much the same the randall aikens link given has to say. or if your handy you could wire it on the jack itself. sometimes your lucky and can skip the resistor and just wire a .001uf / 1n cap across the input jack.
otherwise a 5k and .001uf cap should be fine, allows everything under 30,000 hz to pass and will kill all higher frequency radio interference.