The vent is open to the space above the pressure sensing diaphram. The
other side of the diaphram is connected to the discharge side of the
regulator. Thus one side of the diaphram senses gas pressure and the
other side senses atmospheric pressure, allowing the regulator to
control gas pressure referenced to atmospheric pressure. Normal house
gas pressure is 1/3 to 1/2 PSI above atmospheric pressure. Normally,
the vent allows the diaphragm to flex, letting air flow in and out of
the regulator body above the diaphram, controlling the pressure as the
gas flow through the regulator changes. If the diaphram cracks, tears
or ruptures, gas is vented to the outside, which is not a good thing,
but better than having it vent inside your house.

Call the gas company as the ONLY time anything should come out of that
port is when the load changes which would only take a moment. This
hapens when the furnace or water heater comes on or shuts off. At
those times air will be flowing in or out but only in very small
amounts.

NOTE that when you put your noe right next to the vent, there will be a
SLIGHT gas smell as minute amounts of gas diffuse through the diaphram,
but not enough to make any smell more than a couple of inches away from
the vent opening, and certainly not enough to cause any flow.

Stretch