I come again to get help from the experts.
I am using this BJT transistor in the circuit.
Here is the microphone that I am hooking up in the circuit.
It's an electret condenser microphone from digikey.
Here is my design
I am following the tutorial from this link to build the amplifier. It's exactly the same circuit but with different values to make it work with microcontroller. For that, I have used the supply voltage of 5V instead of 9V as in the tutorial. I have also tweaked the values of resistors to make the correct biased voltage.
The problem is that I get a lot of noise. I checked with oscillosope and it is biased correctly, i.e., midway between the 5V. But you can see a very noisy signal. I hooked the circuit to a computer speaker and you can hear the noise after you hook up the circuit. The output works correctly though, i.e, when you play a sound in the microphone you can hear clearly in the speaker but during idle there is a noise that is audible.
What is the reason of this noise and how to minimize it while the microphone is sitting idle? Thank you for the help.