Hissing sound from my Diff fill plug

I changed my rear diff oil today and as I started to loosen the fill plug it made a hissing sound. Not sure if pressure was being released or if it was sucking air in? The truck had been sitting for a couple of days and I dont do any offroading with it. Is this normal? Is the breather bad? Thanks.

 

My experience on an older Toyota 4x4: Noticed oil leaking at the bottom of one of the brake drums. Pulled fill plug to check level and noted hissing sound like you describe. Wound up replacing axle seals and brakes, and also cleaning the axle breather. The breather appeared to have been clogged. I always did my own work on that truck, and I never had another gear oil leak or hissing sound from the fill plug again. I believe the breather was clogged and caused my seals to fail. I would make that a priority to inspect/clean/replace the breather, you don't want to do axle seals.

 

If you want to replace the breather, might as well relocate it to a higher, cleaner place:
http://www.tacomaworld.com/forum/2nd-gen-tacomas/15686-diff-breather-mod-dummies-pics.html

Instead of the fuel filler door as shown in the guide, I routed mine to the storage cubby near the tailgate.

And did you wait five months to change the diff oil?
http://www.tacomaworld.com/forum/2nd-gen-tacomas/216442-1st-time-rear-diff-oil-change.html