Electronic – ESP32 microcontroller generates noise on microphone

Adding to @GeorgeHerold's answer:

1. You need to include frequencies down to below 30Hz. Most of the energy in a typical heartbeat is between 30 and 40Hz. Given the cutoff for the highpass that @GeorgHerold computed, you are losing a good 15dB at the lower end.
2. Try using a directional microphone instead of an omnidirectional. Directional microphones have two openings, one in front and one in back by the solder pads. I find I get much better results by installing a directional mic such that the front (where you normally speak into the mic) is inside the tube, with the port on the back of the microphone out in open air.

Actually, I don't use a stethoscope in my experiments. A simple plastic shell with a hole in it for the microphone works very well. What I've used for this is a plastic cap like you would use to close one end of a plastic water line. I had some laying around that are about 2" in diameter, and they work well.

I find that a directional electret microphone installed in a platic cap as described works well enough that I can capture heart sounds using nothing more than a PC or smartphone with no additional electronics - I can then do filtering, recording, or other processing digitally.
The PC soundcard has a 20dB boost on the mic input, but the smartphones don't. Since the cards use 16Bit sampling, you have plenty of headroom to digitally amplify the signal even without using the boost. If you resample from 44100 down to 400 or so, then you could probably gain the equivalent of a few bits

The reason why a series switch causes a POP to happen when the PTT is depressed but a shunt switch does not has to do with the DC behavor of a dynamic microphone, combined with charge buildup on the input AC coupling capacitors of the microphone stage.

There is a non-zero current flowing at each input to the microphone amplifier -- this is an input bias current, and is present in all stages that use solid-state inputs. To prevent this bias current from flowing through the mic, and to prevent phantom power from driving the inputs outside the input stage common mode voltage range when dealing with phantom powered condenser microphones, just about all microphone amplifier stages have front-end AC coupling/DC block capacitors. These capacitors charge up to a significant voltage when an open circuit to DC is present (such as a condenser mic, a dynamic mic with a series PTT switch in the open position, or merely nothing at all).

However, dynamic mics, unlike their condenser counterparts, have a very low DC resistance (just the coil of wire). This means that when the PTT switch on your offending microphone is closed, the capacitors can now discharge, forming a transient that creates the POP you are hearing -- the decrease in loudness as you keep using the PTT switch is because the capacitors aren't fully charging before being discharged again.

The reason microphones with a shunt PTT avoid this is because that shunt PTT switch shorts the microphone out when in the OFF position, keeping the capacitors from charging up to begin with.