Signal resonance phenomenon

Hi, I recently purchased an audiomoth for my project collecting ultrasound data.

However, I found that the audiomoth signal resonates in a certain frequency band.

A function generator and transducer(V1000PW21B) were used to generate frequencies.

Firmware: AudioMoth-USB-Microphone-1.2.1

sample rate: 384k, gain: low

no SD card, no AA battery

The data below was collected when the source was 46khz.

75k

85k

I want to know why this signal occurs.

For reference, this signal did not occur when a sensor other than an audiomoth was used

Thanks