Vectra stereo problem?

Hi, I have a problem with my Vauxhall Vectras stereo model number CD30. I can be driving along, put the stereo on and the sound will be perfect but then I get very loud interference, a crackling as if the signal strentgh is weakened. It happens while listening to a cd to so can't be signal related.

It happens on all speakers (used fader/balance functions etc.)

It continues to happen until I turn the ignition off. Even if I turn the stereo off the crackling noise will continue until I turn the ignition off. Once ignition switched off noise will stop until stereo switched on again.

Any info will be a great help