An EGR valve.
The exhaust gas recirculation valve, or EGR valve, is an important component of your vehicle's exhaust circulation system. The EGR valve helps to optimise fuel delivery by recirculating exhaust gases back into the combustion chamber, making your vehicle more efficient while reducing pollution put out by your vehicle. A malfunctioning valve (it sticks) can cause the engine valves to knock against the piston heads (due to improper air/fuel mixture) or for the car to idle rough. Fortunately, checking your EGR valve can be done in your driveway.
Engage the emergency brake. If your vehicle is a manual transmission, put the vehicle in "neutral." If it is automatic, leave it in "park."
Start the vehicle and allow it to warm up to normal operating temperature.
Open the bonnet of your vehicle by pulling on the hood release cable inside the vehicle.
Locate the EGR valve in the engine bay. The location will vary from vehicle to vehicle, so you may need to consult your manufacturer's service manual. In most cars, the EGR valve is located on the intake manifold or head.
Locate the EGR plunger mechanism on the outside of the EGR valve. The plunger controls the exhaust gases being recirculated into the engine by opening and closing.
Have someone rev the engine while you check for plunger movement. You should be able to see the plunger open and close. If it does, the EGR valve is functioning normally. If it does not open and close, then the valve is faulty, and it must be cleaned or replaced.