Air pressure in rear end housing?

[B767capt]

I pulled the rear end plug on my daughters new GT to clean the mag plug and as the plug got loose, air came rushing out like the rear end was holding air pressure. It has 1800 miles on it and the mag plug was in fact covered in fine metal goo which was expected. The car had sat for 18 hours so the rear end was stone cold. I have done this for years on many different cars, but never have I heard air come rushing out. Anyone else notice this? I wonder how much the pressure gets to when hot? I see the vent on top of the axle, but wonder if it's working.
Thanks, Bill

 

[GOLDENPONY]

Rear end

I agree, this does not sound normal. There should be a vent in top of the housing-maybe it is plugged??

 

[NorthwestGT]

There's definitely a vent up there. I think it might be on top of the axle. I remember overfilling the rear on my '96. Fluid came pouring out of that vent. So, you shouldn't be able to build up pressure if that vent is clear.