[patrickt]

If you tap the side of the distributor with the back of a screw driver whle the engine is running does it change the sound of the buzz? That would indicate something vibrating that is making the noise, like maybe something harmlessly loose inside the magnetic pickup. Probably, I would just ignore it. If it's anything serious, it will get worse and then you can just replace it. It's not like it's going to seize your engine or anything.