"Clunk" from front end on reverse over small bump

[mynameisplayer]

Does anyone know of a SIB or known issue - my 50i will clunk when reversing. So far only noticed it over a small ridge at the end of the drive, wheel slightly turned. My brothers E71 used to do it, he would tighten the steering rack bolts and it went away, but would come back. Car is under warranty but I'd rather DIY than leave it with the small town dealer to run over every bump in town to not find the fault. Car is only 20K old, everything looks good up on the hoist.