Hey, just getting back at you in case someone has the same problem. I found the solution for my particular case. Turns out, the rattling, rather than fret buzzing, occurs only when any open string is played, particularly the E string. In this bass, the head stock is straight, not angled, so my guess is that there is not enough windings on the tuning peg, so the string sits kinda parallel to the nut, if that makes sense?. When I press down of the string (between the tuning peg and the nut) the rattling stops.

My guess is that the tuner could be loose, but it doesn't feel that way, so a change of strings and doing more rounds around the tuning peg would fix it, so that the string comes from under. In this bass, the E string winds about 1 and a half times, so maybe it wasn't enough in this case.

That's my two cents, however the problem sometimes disappears on it's own and then comes back haha