Pressure or vacuum build up in rear axle?

I have 17k on my 16 TRD CM Limited. Today I decided to get the break in gear oil out of my rear end and replace with full syn. When I loosened the filler plug there was a hissing sound like opening a cold Budweiser very slowly. Since I dont drink and there was no one else around I surmised it had to be coming from the rear axle. I have never heard a rear end do this before and me thinks this pressure or vacuum could blow a seal. Maybe this is normal for this truck I dunno. I wiggled the cap on the vent fitting and its nice and loose. I have never off roaded this truck and it dont know what mud is so it didnt get plugged by mud..
Any suggestions or experience with this issue?

 

I had mine replaced on my 2010 at about 80K, the dealer did it when it was in for an oil change. They told me if they hadn't replaced it the seals may have failed blowing oil out onto the brake linings. Only cost me 50 bucks. I have a feeling they check it routinely, possibly because it's not an uncommon problem.