So your original complaint wasn't really about the shower head. But it makes me wonder why you focused on the shower head. Did it not hiss before you installed the new shower head? How old is the shower valve?
You speak of a ball, cam and packing. Is your shower valve a Delta. When you had the valve apart did you turn the water back on for a few seconds to flush out the suppliues?
The sound is when the shower is off
Now here's another piece of information that's a real puzzler. Are you saying the only time you hear the hiss is when the valve's turned off? If that's so it sure sounds like you have a hidden leak.
Shut everything down in the house and go outside to the water meter. Check the little pointer. It shouldn't creep or move at all. Get back to me with the results. Good luck, Tom