odiford The power supply can output up to 1700mA
There is a possibility it's operating in a light-load burst mode of operation with Bela Mini. This could be the source of audio frequency noise.
The next thought relates to how closely this noise ends up coupled into the audio path. This is most likely coming through grounds.
Use the scope function on the Bela Mini to see if you can detect the noise at the audio input.
Then step through the system to see where it's coming from:
1) if you stop the Bela sketch so there is no signal being received and send by Bela, is the noise still there?
If there is still noise this would indicate ground differential differences. In this situation you might try connecting ground from then 78M09 and Analog Cirucuit directly to Bela Mini P1.16. so both are referenced off the same GND reference. A further step may be to connect Analog Circuit ground to audio GND from the Bela daughter board instead ofr P16.1/22.
2) If stopping the Bela sketch ends the noise, then you should look at the incoming signal on the scope and see if you detect the noise. Disconnect Audio Out (mono) and connect it to P1.16w/22 GND and see if the noise goes away.
3) If noise in #2 goes away by connecting Auio In L to P1.16, then it would appear the noise is coming from Audio Out (Mono) of the Analog circuit.
The "big picture" of the steps I outlined is to start at Bela output and sequentially disconnect things until noise goes away. This will give us all some hints about where it is coming from and how to get rid of it.
If you parallel a bunch of 9V batteries you could quickly find out if the noise is related to Bela or the 9V power supply.
Switching power supplies are considered noisy: the reason is they generate pulses with several MHz oscillations and fast pulse edges. These fast edges and RF oscillations couple easily through ground loops and even through the air, but the patterns of the energy in the pulses change at audio frequencies. You can't hear a 1 MHz square wave, but you can hear modulated energy bursts of 1MHz square wave turning on and off at audio frequencies (this is why I mention a light-load-burst mode operation as a possibility).
Anyway, I hope this helps narrow it down a little bit.