Over on YouTube user Ancient Discoveries has uploaded a video showing a prototype of his RTL-SDR based metal detector. The metal detector appears to consist of a coiled detection loop antenna powered by an AM transmitter and an RTL-SDR running in Q-branch direct sampling mode.
Ancient discoveries uses SDR# to tune to a low medium wave frequency of around 898 kHz while in direct sampling mode. Then as a piece of metal is moved closer and further from the detection coil the signals on the spectrum move around in correlation with the metals distance. A whining sound just like a real metal detector is also produced by SDR#.