G3's fan is mostly silent. You might hear a small clicking on lower RPMs, and mostly a whoosh on higher RPMS, OTOH, it' still moving air and sucking dust from the bottom.
Looks like K9 needs to move more air since it's packaing an U5 in that small box, so cooling them silently is much harder.
Iirc the N100 idles at 6W and boosts to 25W, whereas the 125H idles at 35W and boosts to over 100W, which the cooler has to be designed for of course. The Ryzen boxes have similar cooling problems from what I hear.
But if you run the N100 flat out, which is likely to happen given that it's not the fastest thing, and disable turbo on this one (which I've done for now since my use case is a low power server) they are actually surprisingly close in heat output. The Radxa X4 tried to cool the N100 like a Pi 5 and it almost melts.
Looks like K9 needs to move more air since it's packaing an U5 in that small box, so cooling them silently is much harder.