I am getting a reasonable output wave from the speaker when I have a slight change in Inductance between the search and reference coil, but I am trying to figure out a good way to eliminate noise when there is a slight difference in frequency between the two coils so that the speaker does not output a sound when there is no metal present across the search coil, as I understand even a slight difference in frequency will lead to a sound from the speaker due to increasing difference in voltage of the two waves. This is the circuit I've been simulating