"Shower tray makes 'noises'"

Maybe it is the water in the siphon/drain under the shower. Indeed, a quantity of water that can slosh around would probably lead to moisture damage on the lower floor. I had such water damage that arose from very little water. If you are unsure, the only help is to drill a little and look underneath with a camera.

Not necessarily. Maybe the concrete ceiling underneath has a proper slope, then it first flows under the screed throughout the whole place and collects there. You can't really imagine it exactly, but if it's a "sloshing" sound, as I imagine the noise, then you already have a real lake underneath and yes, then it should eventually become noticeable under there.

If you're unsure, the only thing that helps is to drill a little and look underneath with a camera.

Exactly, in some way I would get certainty there, otherwise you won't get it out of your head anyway. Maybe you can get in with the camera at a good spot without really causing damage. Take a few pictures and please also report the results.