When I first start driving, my car makes a hissing noise. It is subtle. I used to think it was more audible when accelerating, but it makes it when I am not as well. I have not noticed it while on the freeway (although it may just be too quiet to hear it).
Whether or not the AC or radio is on seems to be irrelevant.
Sorry to hear it makes the noise when you are not as well. Hissing noise is usually associated with a vacuum leak though the most probable cause is a serpentine belt. More info better answers. Try using apiece of radiator hose to pinpoint the source.
Does it matter if it’s a cold or warm start?
If it only happens after the car sits a couple days it could be a thin layer of rust on the brake rotors; this is normal.