[PROBLEM] Rattling noise

[vinnn]

My personal advice is to continue using USB from the Pi to the GD DI-20 going forward.

As well as the clock issues inherent with using SPDIF, the SPDIF interface is unable to handle PCM over 192KHz at 24bit.
The USB interface is a lower noise, higher bandwidth interface and allows your GD DI-20 to natively handle PCM upto 24/384 and upto DSD512 (which SPDIF is also unable to handle).