I would attribute the oscillating nature of the noise to the alternator. But I have no idea how the noise possessed your radio and cruise control.

this might help
Use the 'loaded string applet to help understand the motion of waves and interference.

either you live in an an electromagnetic worm hole, or a combination of bad grounding and uninsulated alternator activity created an electromagnetic field around the chassis of your car. This would explain the possession of the speakers as they are coiled wires and a magnet and require either current flow or an opposing magnetic field to created sound.

any high tension electrical lines around?

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