I have the same issue with 2017 230i. Nice small but powerful car, however when the fuel tank gets below 1/2 this embarrassing noise start coming. The lower the gas level is the more persistent the noise. I wouldn't probably make too much fuss about it if it was a ... 'Yugo, for instance', but hey, it is "Ultimate Driving Machine" for God sake.
So, I went to the dealership three times with this issue. Do not want to go into the gory details, they replace everything but fixed the problem. When I pushed on them on the third time, they finally admitted that they unable to fix the problem at this time. It is an engineering mistake, and 'Engineers' are working on the solution.
Full pants of shit! It sounds like this issue is common to BMW!
The dealership told that it is not 'life threatening', but hey, defect is in the fuel tank! What is it so extra different in BMW from the other cars!
Shall we talk to lawyers?
Vic.