Air Admittance valve CLICKYThis is OEM under my sinks in both bath and kitchen. I stayed with it because of limited clearance above the valve. I change them out at the slightest hint of odor.
Amazon has many levels of quality and design. ACE,HD,Lowes,Menards,Canadian Tire and the like.
They are designed to let air into the drain pipes when you open your sewer lines to dump. If they were not there the vacuum would suck out the protective water seal in the "P" traps.
I don't recommend Leaving the grey sewer open until ready to dump.