It's probably electrical interference generated by the alternator. It might be going away when you activate the brake lights because the current flow changes the regulator turnon point.

You might need to check the capacitor on the alternator to see if it is still good, or add another one.

Do you still have a decent antenna? If you lose enough signal to the radio because of a poor antenna, the radio automatic gain control (AGC) circuits crank up the RF gain to compensate. This will also amplify RF interference picked up by the radio.

So also check the antenna.

Joe

Sorry for the long answer...it's the ham radio operator in me.