I would not be concerned about some hissing when the car is first turned off because the low and high side pressures may be equalizing by the expansion valve. That should go away within 30 seconds or so. If you still hear it, you may have a leak. If you hear it from the center vent, it could be the evaporator. My dad needed that replaced in his E46. Not a fun job.
If you can take it to the shop, they may have an A/C sniffer that can detect refrigerant gasses. Harbor freight sells one for ~$50. If you stick it up near the vent and it goes wild, the evaporator is likely your issue.
'04 330i ZHP sedan | Mystic blue | Alcantara | 6MT | 111k
'98 740il | Schwartz 2 | Sandbeige | 5AT | 238k | lowered on Bilsteins w/ B&G springs
'00 540i sport | Titanium silver | Black | 5AT | 120k
'85 Mustang GT convertible | Medium charcoal metallic | Gray | 5MT | 215k | one owner, all original