Source of Audio Noise at Low Volume -- BlueTooth Mini Amp board
Class D amps are notorious at having really poor power supply rejection. Basically the FETs in the output stage "bang" close against the power rails and anything on those power rails get superimposed on the square wave output AND, to make things worse, the PWM switching waveform actually modulates the power supply noise down to baseband and hey presto, noise at 100 kHz to 10 MHz (or wherever) suddenly appears thru your speakers. It's called synchronous rectification but basically it's a switching mixer like a ring modulator. Here's a picture that may help you understand: -



Imagine the green trace is power supply noise and the red trace is the PWM. The blue trace (significantly magnified) is what results due to switching-mixing and, you should be able to see that it has a significantly lower frequency than either the PWM or power supply noise. Eventually, as power supply "ripple" frequency and PWM frequency start to coincide you get an audio beat frequency. But given there's an awful lot of bandwidth that can be down-mixed to audio there's something always whistling through at audio frequencies.

A series inductor with appropriate current rating in parallel with a 47 ohm resistor and a capacitor down to ground usually stops a lot of this. But to do the job properly needs more info about your power supply and a better idea about the noise on your scope shots.