Welcome to the forum
No noise with the clutch in => it's not the thrust bearing.
If it's happening in neutral, but only with the clutch out, then it must be somethiing to do with the input shaft, or maybe the reverse idler if that is always engaged.
Is the noise still there while you're in gear, or is it too quiet to hear over other noises?
I'd want to get the car up on a hoist, and try to work out the location a bit more precisely from underneath.