I don't think it's the air pump, as the presence of the noise varies with a/c activation (or not). Besides, air pumps make more of a whine than a hiss.

I'm guessing it's the expansion valve, and that he's low on refrigerant. Expansion valves generally make some noise, more when the system is undercharged. It will hiss more or less with changing engine RPM. After a minute or two the evap will have settled down at its minimum temperature, the expansion valve will cut the refrigerant flow, and the noise ceases.

- JimY