I am sensing audio with an electret micrphone and I have realize that the photon detects a peak periodically, about 1 minute between each one.
If I connect the WiFI external antenna, the peak reduces its amplitude a lot, despise it still remains…
How could it be possible?
Any high-gain circuit can be affected by noise. The noise could be coming from the RF directly, especially if you are breadboarding and have long wires or wires near the on-board antenna, or the noise could be conducted from the power supply since WiFi transmitting requires increased power, the voltage you are likely using from the Photon drops briefly.
You should try to make your external circuit as neat as possible and avoid the antenna area. You can add capacitors to your circuit to try to limit the conducted noise–using both small value (0.01 to 0.1uF) and large value (like 22uF) electrolytic caps will help. If you have a really low-noise application, you may need an isolated power supply for your microphone and extra shielding to block the RF.
If you can post some photos of your circuit, we can give better advice.