What is this apparently A/C-originated hissing sound?

I recently got my car back from having the transmission replaced.
The car seems to be running fine, but I hear a low humming noise every now and then.
It happens when the air is on and it continues to make the noise after I turn the A/C off and after I turn the car off.
I was told it might be the air compressor… but I am not sure.

High pressures build up in the A/C lines while operating. The pressure builds when the coolant is being compressed.
If the A/C is cooling well, it probably is not leaking while running. If it were leaking during operation, it would soon not provide cold air.
If it makes the hissing sound for only a minute or so after being turned off, it is likely that the A/C system is simply normalizing.

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