pops and ticks from DAC

Bluesound Node 2i plays fine streaming and off USB drive through its own DAC and over bluetooth. Oppo-105 plays fine from FireTV, USB drive, or disc. Node 2i into the coax or optical input of the Oppo-105 produces random pops or ticks. Could happen several times in one track or not for hours.
How do each of these inputs or outputs come together? If the Node2i is the issue (still under warranty) then why only on the digital out? Wouldn't this digital signal be the same that is fed into it's internal DAC? If the Oppo is at fault then is there a common input buffer for the digital input that would effect both optical and coax but not the internal sources from the Oppo? Not knowing (or completely understanding) the internal signal geometry makes it harder to troubleshoot.
The easy thing would be to try another DAC and see what happens, but I don't have one to try. I've been thinking about upgrading my DAC but have been spending money in other areas first as the Oppo DAC is pretty good. That was one of the reasons I bought it in the first place. I don't want to waste money on a cheap DAC as a diagnostic tool. If the issue is in the Oppo S/PIDF then I guess I'll need that DAC sooner. I had been leaning toward a Chord Qutest, but that isn't in the budget this week. Would a Schiit Modi be a downgrade from the Oppo? I don't want to buy it with the intent of returning it. Maybe a Bifrost would be a lateral move or slight upgrade?
I'm not sure when it started, and sometimes I think I'm imagining it, but I've been doing more critical listening as I've been revamping the system lately. Intermittent issues are hard to diagnose. I've tried running the Oppo in pure audio mode where it shuts off all video circuitry, and that seemed to help, but I wouldn't have a clue how or why and it only occurred to me lately so I don't have much time listening in that mode. If you change something and it doesn't immediately repeat that doesn't mean it won't. All I really want to do is listen to music.