power supply is 9v 1.3a, and there is not much current draw from the modules, so I think this should be ok.
'separate power line/ribbon' , so the AE modular, has a master module, which takes the 9v PSU and then (among other things), has two ribbons from it, one feeding top row, the other the bottom row - so the noise only appears on the same row as the bela mini is on.
I could possibly power it separately but that's not ideal (but I do mean to do this as a test anyway) as two things to plug in... though if powered from the usb, then this obviously is useful for programming too.
this means drilling the case, and also 'fixing' the module position - not really a big deal.
(unless I do like salt, and get a small extender to put on the front panel)
another option, would be to look to take the power supply from the 9v supply already present, and the put thru a regulator down to 5v but again this kind of fixes the modules position, and is a bit more complex.
Ive asked Robert, and he thinks perhaps an inductor , which also someone on the muffwiggler forum thought too, but the size of this needs to be calculated.
@giuliomoro, do you know how this was done for Salt? was there a similar issue?
( unfortunately terminal tedium (rPI eurorack) don't have a schematic so not sure how they solved the issue)
small update: (more info)
so I tried powering the belamini from usb (just taking gnd to make it common) , and viola the noise goes away.
so definitely power related.
I had a quick look at the terminal tedium, which i think is similarly taking 5v from eurorack to supply a rPI3, and whilst i cannot find the schematic(!) , I can see on the BOM they use an 10uH inductor (mouser #542-78F100-RC), but without the schematic I cant see how its connected, or if its used in conjunction with something else (capacitor) - or 100% confirm it is using the 5v supply (but does look like it)