I finally had time to come back to this set-up.
When the speaker is connected, Bela is nice and silent when building a patch, but get's this buzz when the patch is running. This happens with multiple patches.
Also, the buzz is in the headphone output when the speaker is connected, and is gone when the speaker and speaker power aren't connected.
And what's weirder: in my headphone left and right channels are fine, while only the left speaker plays sound and the buzz, and the right speaker plays very loud bursts of the buzz, in a rhythm not unlike the flickering of the blue LED's.
here's two videos: https://www.flickr.com/photos/62566466@N07/shares/3HFVU7GS60
PGA Gain settings don't make any difference.
Mute speaker gets rid of all sound including the buzz through the speaker AND through the headphone output.