Rear differential hissing

Okay this was really weird. I'm not sure if anyone else has had this happen to them,
so I thought I would post it and find out.

I was in the process of changing my rear differential oil and removing the fill plug. After breaking the plug loose and turning it to remove it, it started hissing like there was pressure or a vacuum inside the differential. I have never seen this happened before but admit I haven't change differential oil that often. I did check the vent after draining the oil and it looks fine. Just wondering if this is normal?

 

Entirely possible. If the air trapped in the diff became hot it would expand creating pressure, and you relieved the pressure when you open the fill plug. That said, the air should not be trapped in the diff.

In the photo below, the stem sticking out of the axle housing is supposed to allow the diff to breathe. You can see where mine spit up some diff oil, probably when off roading on bumpy roads in the desert.

Maybe yours got plugged up?

 

Yep, wait too long and it might take out an axle seal too.

 

Thanks guys I'm going to pull that breather out tomorrow and have a closer look at it.

 

So I pulled the breather out today and the same thing happened. The rear different is under a vacuum.
So I pulled the cap off the breather and found this.




It's a check valve.