Cabin air recirculation fan? Not sure where these are on the Neo, but maybe one has got damaged fan blade(s) causing the sound?
Something in the galley - have you tried switching off the galley(s)?
Aircon pack? Have you tried turning each pack off one at a time?
Door seals can be extremely noisy too and can make what sounds like a mechanical noise.
Cabin over or under pressure valve?
Could even be a faulty electrical power supply for a light fitting resonating against something.
Just ideas - obviously hard to assess without hearing the actual sound.