Have you spilt milk on it…snap, crackle and pop. Sorry I just couldn’t resist.
I have a Pi4 with the DigiOne HAT, via coaxial I have not noticed this. I’d take the Pi4 and HAT apart. Ensure no dirt or grease on pins. Ensure assembled with bridges between the boards. Swap power supply (brick and cable). Also worth a try is Volumio, which I run. Also, try a different coaxial cable.
Do you have another streamer you could connect to you DAC/headphone amp to eliminate these.