Asymmetric Room

@OP - simple test, if you are getting the same resonance a foot away from each speaker, it's probably not the speaker. If it's only one speaker, then a speaker drive unit/crossover is more likely.

But also check that there isn't something rattling in your room e.g. ornament on a shelf etc. It does happen.

@yoyoyaya both speakers

On a positive note, I got my 3rd hole in one yesterday, so at least my golf game is working. 😒

Playing golf this morning, so will try some things this afternoon & report back. Thanks 

When did this start? Do you recall what change triggered this issue? 

@audphile1 Great question. Been thinking about that myself. I think the answer is it has always been there. Like all of us, I have been making continuous improvements and the occasional hash is more noticeable or all that is left. I ran the frequency sweep, proving the perceived issue for the first time a few days ago.  
One recent change is I had a smart switch on my Qutest to allow for remote power off that I removed. I suspected the smart switch was introducing post conditioner power supply noise.
However, I found when the Qutest is left powered on, it begins to introduce hash. I manually powered off my Sbooster and both perceived & measurable sound improved, not perfect, but better. 
I will either have to manually power off my Sbooster or bite the bullet and purchase a different DAC. I think I know what your vote would be. 😏