Air Conditioning Hissing and Fix

In my 2004 330Ci ZHP with 88k miles:

I started to notice a hissing sound when my A/C was turned on, which was especially noticeable right when the car was turned off or when the A/C was switched on or off. A/C was still blowing cold. Adjusting the fan speed didn't affect the sound, and the sound would disappear when the A/C button wasn't activated.

The sound was similar to the high pitched sound of a CD drive. I took this into my local shop, and had them run an A/C diagnostic. They recharged the A/C (with dye) and noticed that it was little low. The diagnostic and re-charge cost around $200, though this includes the next appointment should there be an issue.

Low and behold, it turns out that this recharge solved the problem (for now). I will keep an ear out for the return of this sound, and if so, we now have dye in there to see where the leak in the A/C system is. Really hoping it isn't the evaporator and that it was as simple as a low charge.

Yep, the hissing is the first thing I notice when the refrigerant gets low. The next time it does it, note the temperature difference between the left and right sides of the center vent. As you lose refrigerant, the left most vents in the car keep pumping cold air while the right vents get warm.