just running it now on my analog four. Its running smoothly, quiet. Cant hear any fan. There is a ventilation on one side which might be a fan, but cant really tell. i can say the first 15 minutes its running silently.
Edit: OK been running it for half an hour now, its warmed up, and is making some noise, relatively subtle., no interference on the synth audio as far as I can tell, but you can hear it when no music is playing just in the general room.
Edit 2: OK I think I can hear a very slight hum in the audio of the synth. I know its there because when i put my hand on the battery, the buzz is quite loud. When i take my hand away, it disappears.
So yes if this kind of thing bothers you then don't get this battery. For me its Ok to use as a portable setup but when recording will most likely need to switch to mains.