I've built up a little stereo speaker system with USB-powered speakers(just powerd by USB, sound still comes from the 3.5mm jack) and the RPi 3. When I use the speakers on a seperate outlet everything works just fine, but when I connect them to the RPi3's USB, I get a constant low frequency hum from the speakers. I've seen the same problem when using the same external battery pack for the RPi3 and the loud speakers.
What is this noise? From other sound equipment I'm used to the mains hum when something is not built properly, but I'm not sure how the RPi could create this noise when the connected parts are both DC powered..!? Is this an electrical/engineering problem of the RPi? And more importantly, is there a way to get around this?