I plugged in my RPi B+ (with latest Raspbian) to a speaker (August SE30, using the analog jack). I get the expected sound when using
mpg123 but I also get, when not playing, a continous stream of noise which shadows the RPi activity. You can hear how it changes during the boot phases, then with any activity (downloading something, actions on the SD, etc.).
I tried a different power source and signal cable. It does not seem to be related to the issue which was fixed with a recent firmware (I upgraded everything). The bluetooth audio connection does not have this problem (but I would like to avoid using it for other reasons).
My question is whether this noise is expected (for instance due to some interferences between the analog converter and some other element in the RPi, or any other known reason) or if it should not be the case.
If it is expected then I will invest in a USB audio card, but would prefer to be sure beforehand.