A metal detector used for identifying contaminants in packages on a conveyor. The detector includes coils a search head and an analog to digital converter generating a reactive signal and a resistive signal in response to the presence of a contaminant. During a learning mode a sample product passes through the metal detector providing a representative product effect signal which is stored by the reactive learn memory and the resistive learn memory. The learn memory values provide a reference value subtracted from each product signal during a normal production cycle, canceling the product effect caused by contaminants in individual packages. The product effect is monitored during successive cycles composed of a series of packages undergoing normal inspection by the metal detector. A tracking processor averages the product effect signal produced by the individual packages and continuously updates a product effect trend signal that is subtracted from each product signal.