Like some others, I had misgivings about the sudden very loud fan noise due to the high rpm in the Eco mode. However, I have a watt meter in my possession, so I decided to observe the reality behind it, at least.
I just built a new Ryzen 3700X PC, still using my old Radeon 390 video card. While normally the Windows desktop idle power use (of the whole PC, not including monitor or such) is around 43-51W, I noticed that when browsing web pages on Firefox, the power use can jump to 150W or more. Since 20% of 750W is 150W, that would indeed explain why the fan would periodically kick in the Eco mode. This is naturally quite distressing as the sound is very audible over the steady hum of the rest of the PC, which does have multiple fans, but with adjusted fan curves for silence. The same naturally happens with some other tasks, like watching videos, especially fullscreen.
My previous PC was with an i5 6600K and Supernova G2 750W. On that PC the Eco mode didn't produce noise issues like this. However, due to having no issues, I never measured the power use either. Since the cables are compatible, I could theoretically test the G2 on this PC as well, but I'd assume the Eco mode would work identically, the audible results thus differing only due to the differences in the fan.
My only option is, more or less, to keep the Eco mode off (unless a new video card eventually lowers the power consumption sufficiently). I have a bad feeling I'll need to eventually seek a fan replacement.