Probably not of interest to that many people, but….
Using volumio with an i2c DAC in a classic car that has a dynamo (producing the dirtiest 12v known to humankind) gave me the worst sound imaginable when the engine was running.
I tried many in-line filters to clean up the dynamo 12v output but none made enough difference.
In the end I decided to clean up the 5v supply for the pi alone - with one of these.
Success!