I have a stock 512k (fanless) and a 512k with a fan that accompanied the internal 20MB GCC HyperDrive that was installed in it. The HyperDrive itself is pretty loud, so you can hardly hear the fan over the drive noise. But if the drive isn't spinning, you can definitely hear the fan. For that very reason (fan noise disturbs my mental health terribly), I swapped out the rather loud Elina stock fan in my SE/30 for a hardly audible Silenx 60mm fan. But if you choose other Silenx models, beware. I bought an 80x25mm Silenx and an 80x15mm Silenx to use in my G4 Cube. Both 80mm fans are either defective or Silenx simply ripped me off in that they are louder than the "loud" Elina fan used in my SE/30! But again, I can say that I absolutely love the 60mm Silenx I have in my SE/30, and if I were to put a fan inside my fanless 512, it would certainly be another one of those! The 60mm Silenx also draws a mere 50mA of power, so it won't tax your analog board.
The question at this stage is how to mount a fan in a 128/512 since, unlike the SE series, they don't have a ready-to-go fan slot waiting for a fan and screws. My HyperDrive machine uses velcro, of all things, to mount the small fan in the upper left back corner of the machine, shooting hot air out the slits in that part of the case. Even after all these years (since 1985) that tape has kept that spinning fan in place. So I guess if you can find the same quality of velcro tape, that would probably work just fine. But I would also think that mounting a fan in other ways could work just as well, even mounting the fan to blow air directly onto the analog board. Indeed, any air movement inside the Mac's housing would make it cooler than no air flow out all. But then, you should open such Macs up every year or two to vacuum out the dust bunnies sucked in by the fan. Your floppy drive may need cleaning more often too.
The question at this stage is how to mount a fan in a 128/512 since, unlike the SE series, they don't have a ready-to-go fan slot waiting for a fan and screws. My HyperDrive machine uses velcro, of all things, to mount the small fan in the upper left back corner of the machine, shooting hot air out the slits in that part of the case. Even after all these years (since 1985) that tape has kept that spinning fan in place. So I guess if you can find the same quality of velcro tape, that would probably work just fine. But I would also think that mounting a fan in other ways could work just as well, even mounting the fan to blow air directly onto the analog board. Indeed, any air movement inside the Mac's housing would make it cooler than no air flow out all. But then, you should open such Macs up every year or two to vacuum out the dust bunnies sucked in by the fan. Your floppy drive may need cleaning more often too.