My left speaker emits a slight soft static high pitched noise

Hi all, 

I started noticing these last several months (or perhaps it's been there forever I just never paid attention before until these last several months) that there's a slight soft static screeching high pitched noise coming out of my left speaker only when I put my ear right up against the tweeter and could sometimes be heard from less than a foot away if my ear was right up against the tweeter. But I can't hear it at all from my listening position (my seat).
The noise wasn't audible at all when music was playing even when I put my ear right up against the tweeter of my left speaker. It happens only on the left speaker. But it doesn't affect sound quality at all. And again the noise is very soft emitting, you would have to focus on that and you will have to put your ear right up against the tweeter in order to hear it. My listening room is very quiet and small. 

I have a cheap on loan Comcast Xfinity internet Wifi modem/router sitting right next to the left speaker since the internet portal is located right by the left speaker. I have a small listening room and very limited space. So, I gave no choice. So, is that maybe a cause of that slight soft static screeching high pitched noise coming out of my left speaker? And again it's only my left speaker which is literally right next to that cheap Xfinity Comcast internet WiFi modem/router. But again it doesn't affect sound quality at all. I think that slight soft high pitched noise might have been there since the beginning of time, I just never paid attention to it until several months ago. 

Or it could have been an AC related born noise caused by noisy AC line in my old multiplex house? I live in an old multiplex house in urban neighborhood of Seattle, WA and I believed the AC line is very noisy. I'm using a Shunyata Research PS8 AC power line distributor with the Shunyata Venom Defender plugged into one of the outlet on my PS8. My AC power cords consist of Shunyata Delta NR (2), Transparent Reference MM1, PS Audio PerfectWave AC-5, Audioquest NRG-4. 

I have pretty new speakers. I bought them new last year. They are Monitor Audio Gold 200 5g (the latest version of Gold series from MA). they just came out last year (2019). The rest of my associated components are : Classe Sigma Amp2 class D power amp, Classe SSP 800 preamp processor, Cary Audio CD306 SACD player/DAC, Oppo 105 universal bluray disc player. I'm using high quality interconnects (XLR balanced analog & some digital cables) and speaker cables. 

So, what would be most likely the cause? Again it's only on my left speaker and it isn't detracting from sound quite at all and it is not audible at all from my listening position. I will have to put my ear close to the tweeter in order to notice it. I haven't experimented swapping out the left & right speakers, just in case it wasn't the speaker. What do you guys think the most likely cause? Is it the power amp, preamp processor, bad speaker's crossover, AC related born noise or that cheap on loan Comcast Xfinity internet WiFi modem/router which is placed literally right next to the left speaker since the internet portal is located right next to the left speaker? 

I'd really appreciate the inputs, thoughts and suggestions. 
Thabk you in advance.