Hi,

What is considered normal is to have the gas to make no discernible noise or if you do hear anything it will be a slight pressure release sound.
The cap is meant to trap gas vapors in the tank and its evaporative emission control system. As mention the canister is up under the front bumper drivers side.

There is a check valve on the canister operated by manifold vacuum under certain manifold vacuum conditions. If that is working it should under most conditions keep the fuel tank very low on pressure.
During hot days or excessive agitation of the fuel (low on fuel) there can be times that the canister does not keep up with.
This is when you might hear a eek-sound to almost a chirp.
This is the gas cap leaking off down to a preset pressure.
The gas cap also lets air in so a vacuum is not created and this works both ways when changing altitudes, like traveling through a mountain range.

So, an occasional pressure release is very normal.
I think it maybe mentioned in the owners manual or has been stressed by Volvo service bulletins.
In fact, if it didn't do it occasionally, it might mean that the outer split rubber o ring or the caps check valve inside maybe leaking.

I suck and blow on the cap to make sure it holds but let air in!
I can't get it to pressurize high enough to to make it release. So, it must be a few psi or more.
The outer ring is a visual thing, looking, for cracks or listening for that pressure noise.

Phil