[Motor31]

You might have another cause for it too. Unfortunately it will require removal of the bowl to fix. The gaskets may have shrunk or failed a bit allowing gas to re enter the unit. This would be the gasket around the base of the toilet drop tube. In a stick home, like the wax gasket the stool sits on. The bowls flapper valve (rotating valve) will still hold water but the gasket is farther "south" of there.