Low noise electret mic circuit
PC power rails are notoriously noisy. Here's the rest of the image that you used in your question (from here):



Note that any noise on the power rail is coupled directly through the bias resistor to a high-gain audio input. I would recommend adding additional filtering:







simulate this circuit &ndash; Schematic created using CircuitLab

This creates an additional low-pass filter for the microphone bias voltage with a cutoff frequency of about 7 Hz.