I had heard about the humming, and went with the universal MSD one instead of the mustang specific one. It's mounted on 3 little rubber pieces and doesn't hum at all. It was also $10 cheaper if I remember right. Just a suggestion for anyone looking to buy one.
I put a Crane Fireball ps91 coil on with my MSD6al and have had no problems. The guy @ my local speed shop told me not to buy the msd coil because they've had alot of them come back. Just fyi.
"The frequency in which the coil operates can cause it to buzz. To help reduce the buzzing install rubber washers between the coil and the mounting brackets."
I rubber mounted from the coil to the bracket, and from the bracket to the fender....it's still buzzing. There is actually nothing wrong with it, they just make some noise. You get used to it real quick - it's not bad at all.