Well, this post will probably wrap this up. It is definitely the powerline adapter. I just received an isolation transformer that I was hoping would fix the problem (as of the time I received it, problem was still unknown. Assumed to be dirty power). It actually suppressed a lot of noise, but not all of it. So really more of a band aid than solution. The a-ha moment came when I simply unplugged the powerline adapter completely from the wall. In all my troubleshooting, I hadn’t done that before. Don’t ask me why since it was the simplest thing to try (I think I assumed it was dirty power or an equipment issue as many of you did). But as soon as I did that, noise gone. Long story short, it could be the general powerline adapter technology, or just this one (TP-Link AV2000) but obviously they have the potential to be problematic. However, I have read other audiophiles who use these and have good results, so it looks like it may be a case-by-case basis, depending on house wiring variables. In fact, I used another set of adapters in a different house and didn’t have any noticeable issue. YMMV.
I spent around $200 for the transformer. I suppose I should return it and use that money toward a decent MESH system. Thanks all for the troubleshooting help. 
BTW, I googled “TP-Link AV2000 Powerline Adapter noise” and got this result top of page – exactly what I was experiencing. Someone else posted a YT video showing same (also with my exact unit, so perhaps there is something going on with the TP Link)