So I pull the truck into the garage to address my rear end fluid leak into the emergency brake area and I hear a hissing noise (like a radiator cap expelling antifreeze)which slowly gets quieter and quieter. So I turn the truck back on for 40 sec then off and try to find where /what it is. Seems to be coming from passenger side engine compartment. I did notice the a/c expansion tank was cold. Could the noise be from the turbo vanes slowing down? Or does the a/c make this kind of noise? Any help would be appreciated

 

Noise is normal after running a/c and engine is shut down, just the pressure of freon gas in A/C system equalizing across low and high side of orifice tube in a/c system.

 

Thanks - I must of had the defrost which made the a/c go on. I was nervous just because I never heard the noise before