Mitsubishi Challenger Differential

Its normal. It a bit of air rushing in. The air in the diff expands when hot and contracts when cold. It tells you that your seals are working perfectly!

Exactly what Phil said...

Yes it is a sign your seals are working but more importantly it is also a sure sign your diff breather is blocked. As has been said when the diff gets hot the air inside expands and has to be able to escape to equalise the pressure with atmosphere otherwise the pressure build up would blow diff oil past the seals. Also when the diff cools down the air contracts and if weren't for a functioning breather air would have to be forced in to the diff by the higher atmospheric pressure outside the diff. It is particularly important to have the breather functioning when you do a water crossing otherwise the hot diff will suffer water ingress past the seals when the air inside the diff cools rapidly upon entering the water. The breather is probably blocked with mud, dirt, dust, wasp nest, etc. Look for a small tube or hose from the top of the diff or diff housing going up to a high point under the car. Remove it, clean it, blow through it and re-fit the line and you troubles will evaporate. Same applies to the front diff and gearbox/transmission.

If there is any puff OUT, or IN, the breather is blocked and needs to be checked. That is why they have breathers, so they can "breathe". Ie pressure equalization with outside.

There should be no noise as you described and if I ever hear any, I check the breather hose for being clear AND also check the fitting which attaches to the axle housing.
It is usually a small dia hole in the fitting and dried oil scum. especially if a diff additive is used will dry in there and cause a blockage.
If any pressure builds up, the pinion seal is the high speed seal which will be stressed with higher lip loading than normal and it will just wear away.
Landcruisers will do a seal in 200km easily because they are faster than the wheel seals, but the wheel seals are also suffering and thier life shortened considerably.

If thee has been pressure and it gradually escapes AND the seal lips then holds a vacuum, when crossing rivers etc, the sudden cool will increase the vacuum and surely suck in water.

Having a breather is the best way to stop it all happening, Therefore, there must be NO FFFTT when you open the filler.

So, sorry, I can't agree with anyone who says it is normal or ok.

Exactly Phil...Ross is wrong but too bloody minded to admit it. Seals can take a fair amount of pressure and as you say Toyota have been doing it this this way for decades. The valve is simple and efficient and WORKS...