HEI in a Mopar?

Not to interrupt you guys, but I've noticed something since I drove the car yesterday and today (weather's been nice), I detected a very low intensity electrical noise-not through the stereo, but if I lean over the coil and listen, I can actually hear it firing...never heard anything like that until I installed this system...almost like a 60 cycle hum, if you will...but I can only hear it below about 1600rpm or so...after that it's pretty much drowned out by other background noise...Del, you ever notice that?