@asddsa, you are right on! Thanks!
I tried your idea. I used a USB wireless adapter to by pass the RPi 3 built-in WiFi. And the result is satisfactory, i.e., the clicks have much much reduced! And I am noticing much higher WiFi signal in Volumio’s network page. I guess by using an external USB WiFi the load to the Ethernet/USB controller chip has been reduced and therefore clicking noise is reduced.
P.S. I also did a stress test: rescan the library while playing back. And it does not cause any problem! (When I use the built-in WiFi adapter and let Volumio do the re-scan, the clicking noise is unbearable!)
P.S. again: After very careful listening, I conclude the clicks are still there but it’s very mild and subtle. From my perception, it’s like very mild pops from vinyls. It does not bother me much. But it’s just perfect. Maybe, for the next step, I should just switch to SPDIF. Asus Tinkerboard looks very good from spec. It has separate USB and Ethernet controls. Also it has native 96K/24bit capability. Does anyone have any success story?
So much for my excitement on using RPi has a high end music playback device. Music is about pursuing perfect. RPi is limited by it’s weakest link…