Ya: Ethernet to Fiber media converters
It’s one of those things where you don’t realise there’s noise in your system until you slowly remove each source of noise.
The powerbanks are great in that they are off the grid but they are also higher noise - over 300mV of noise for the TP-Link power banks when I asked TP-Link. There may be lower noise powerbanks out there though.
You may (or may not) hear even more improvement if you feed the downstream FMC with a lower noise PSU.
But no rush - I found the improvement is almost immediate even with the stock PSU’s that come with the FMC’s so there’s no rush. Something to consider later maybe?
And I found keeping the downstream ethernet cable as short as possible helped too. I got 6-inch Blue Jeans Cable Cat 6
In the mean time, enjoy !