Hissing noise with GE Profile refrigerator

Hissing noise with GE Profile refrigerator I have a GE Profile refrigerator PFE24HSLAKSS model, it is very noisy when working. A hissing noise comes from the bottom of the freezer and I noticed when I open one of the two refrigerator doors, the noise from the fan decreases. When I shut the door the noise starts again. The fan has been changed 2 times.