In a two-channel magnetic metal detector for a forage harvester, positive and negative adaptive detection thresholds are varied according to long term changes in the noise component induced in detection coils by moving ferrous parts of the crop feed mechanism. The adaptive detection thresholds are set at levels determined by the positive and negative peak averages of the noise components in the channels while the crop feed mechanism is running at normal speed. Start-up transient noise induced in the detection coil as the crop feed mechanism accelerates may exceed the adaptive thresholds as thus set, thereby triggering false indications that a tramp metal object has been detected in the crop feed path. At start-up of the crop feed mechanism, the detection thresholds are raised to prevent false indications. An increase in a channel noise signal is taken as an indication that the crop feed mechanism is accelerating. The detection thresholds are then raised to a level above any transient noise level expected during acceleration of the crop feed mechanism. After a predetermined time sufficient for the feed mechanism to reach normal operating speed, the detection thresholds are set at their adaptive detection levels.