Problem might be the rear shocks with PN 1015620-03-C, the 1015620-03-B are smooth, i don´t know if the later 1015620-03-D are too. the 1015620-03-C are hard, causing the hatch buffeting.
I tested a 2016 pre FL 85D with a very loose hatch and there was no booming, it had the 1015620-03-B, nice...