Negative Resistance Wand
L2 is a single turn a few inches or cm in diameter. Adjust the size until there is no noise from the speaker. Or adjust R1 similarly.
Bringing the negative resistance wand into proximity to a resonant circuit (L1 C1) will cause it to oscillate due to coupled negative resistance. C2 causes an audio frequency super-regenerative effect that can be heard in the speaker when L1 C1 oscillate.
You can tune a radio receiver to find out at what frequency L1 C1 are oscillating at.