CB engine noise, MAN 440

Get a large value capacitor and fasten the lead to the output of the alternator and the body of the capacitor to the body of the alternator.A old type points "condensor "will do off some old Ford or BL car.You can still get them from a decent auto factors.

It allows the varying output voltage a path to earth to stop signal hum running through the truck wiring loom caused by an unsmoothed (in comparison with a radio signal)alternator output.The capacitor will allow a varying signal to dump to earth but will block dc flow.