Hey, thanks all for your input. Highly appreciated!
*Does the noise vary with speed / altitude ?
-> No, it didn't. Wind noises during flight where masking/covering it, but it remained audible and steady. That's why I was particularly interested in what kind of generator is placed under the cabin floor. Airpumps of some hydraulic reservoir or so.
*Check cabin door seals. Check toilet / galley sink drains.-> As per my uneducated guess it was not pressurisation related, more (quite surely) a mechanical induced sound. In case you've driven a manual transmission car in reverse gear, think about the sound emitted when going backwards at a high speed.
*Is it only noticeable with engine(S) running?What was the action taken by maintenance when you reported it? -> Yes. We're not sure when exactly it started, but with only GPU/APU running it was not there. Maintenance was of the opinion it's more noticeable on the NEO compared to the CEO because of the otherwise reduced noise pollution but I kind of disagree. Even if so, the tech nerd inside of me would still be interested in knowing what it is. 3 days later the ATL neither shows any rectifying actions taken nor any additional writeup by some other crew.
*Am I correct in inferring that the issue only affects one airframe from your Neo fleet ?
-> Yes and no. None of the CEOs have it. The one NEO has it, and it's persisting as to our WhatsApp group. Since it's the only NEO in the base so far I unfortunately can't elaborate further.