Actually, you want the oscillators running at different frequencies, such that the difference between their respective frequencies (the beat) is about 400Hz. The search oscillator may only be pulled a few Hz as the coil is moved near metal.
The frequency response of the audio amp and speaker will not reproduce anything below ~50Hz, so you need to make the initial beat ~ 400Hz, which you will be able to hear in the speaker. Turns out that human hearing is very sensitive to pitch change of a tone near A440 on the musical scale.
If you set the oscillators to almost the same freq, they might injection lock to each other's frequency due to inadvertent coupling (magnetic or through the power supply), in which case you will hear nothing...