There is a Air Admittance valve under the sink. It's usually black plastic and screws on by hand. The valve acts as a vent to allow water to drain. When you put water in the drain the valve opens. If it stays open it allows gas from the grey tank to rise into the room. The black plastic valve are cheap and often fail. You can screw it off, cover the pipe with something, and take to any large hardware store or home. depot for a replacement.
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