Other thoughts:
Bad spark plug wires can be a source of intererence, or loose plug wire connections at either end. Loose or corroded ground connections could be a source. Loose connections in any of the ignition components can be a source. A regular popping noise that speeds up with the engine is probably ignition related.
The factory installed interference suppession on cars equipped with factory radios, but if the car was built without a factory radio it may not have the suppression parts. For example, the ignition coil on my car has a capacitor connected from one terminal to ground to supress interference from the coil (not the capacitor on the distributor, but looks similar). I think radio noise suppression kits exist if your car doesn't have the factory stuff and you can't find an obvious source.
George Davis
'69 280 SL Euro manual