Here the very simple and easy build gold detector circuit. The circuit capable to sense gold or metal or coins from a distance of about 20cm, depending on the size of the object to detect.
The circuit oscillates at about 140kHz and a harmonic of this frequency is detected by an AM radio. You can simply tune the radio receiver until a squeal is detected.
When the search coil is placed near a metal object, the frequency of the circuit will change and this will be heard from the speaker of AM radio.
Below image is the construction of the circuit, you will see that the radio is placed on the hand stick of the complete detector.