Recently, my radio began receiving interference, presumably from the engine, however the interference increases in frequency with the speed of the car,not the engine rpm's. I've changed the plugs, plug wires, distributor cap and rotor and all body ground wires, yet the interference persists. The alternator was also replaced (for other reasons) and the replacement had no effect on the interference. The battery was also replaced after the interference began with no change noted. The radio itself was installed about 15 months ago and worked fine previously. Needless to say, I am open for suggestions.