I have tried RME ADI-2 Pro with Volumio installed on RaspberryPi 2. Installation was easy and I do recieve good quality audio output but also there are clicks/jitterinng I would say every 5-10 seconds. I have tested multiple formats and both wired and wireless connection. RME DAC is perfect and works flawlessly with both Windows PC and Android phone connected over OTG cable.
How can I debug what the issue is?
I have an RPi 2, and tried to use it with Volumio. It worked fine with CD-quality files, but was unable to keep up with higher rate formats such as 384 kHz/16-bit. Requiring 100% buffer full before beginning playback simply delayed the issue a little. I also have an RPi 4B, and it handles everything perfectly. It is possible that your RPi 2 is not keeping up. I suggest adjusting the buffering to see if that changes the timing of the clicks. If it does, it supports my hypothesis, although not being definitive.