I did some measurements of the HiFiBerry DAC for the Raspberry Pi. Compared to the DAC in standalone mode, there was noticeable more noise when I connected the DAC to the Raspberry Pi. Ok, that was expected. There is a lot of digital switching on the Raspberry, this is not good for audio circuits. But then I noticed an interesting fact: Even if the Raspberry Pi was powered off, there was still some noise.
Wow! Silence! The 750Hz peak was a problem in our measurement setup.
But where does this noise come from? When you connect the Raspberry Pi do an ethernet network, the data connection is galvanically decoupled using a transformer. However, if you use shielded Ethernet patch cables (STP), the ground of the Raspberry Pi (and your audio equipment) is connected to the Ethernet network. This is where all the noise comes from. The solution is simple: You should use unshielded twisted pair cables (UTP) to connect your Raspberry Pi to en Ethernet network or use WLAN.