The gas will sometimes make a sound as it moves through the line to the burner but the actual meter should not hiss. Take some like spray cleaner, a soapy kind, windex even and spray around the meter if you see bubbles then there is a leak otherwise it is the sound of the gas moving and not a leak. The bubbles if they appear mean you need to shut off the gas and secure the connection as this is a highly dangerous issue.
As count vac said, gas as it flows through the meter and regulator that is next to the meter do make a hissing noise and the more appliances drawing gas, the louder it will be. If you hear it but don't smell it, chances are its just the gas passing through but if its audible and you can smell it call your gas supplier. Do not spray any household solvent on the meter as they contain chemicals that can corrode copper and eat away at the gasket materials. Call the gas company and they have non corrosive soap that will bubble or a gas detector.