Rear Differential issue

[aseq]

Hi, My M40 318i has a strange 'whooping' sounds as if coming from my rear differential. My mechanic thinks it is not from inside but where the rear diff. is connected. Has anyone experienced this before?I have been looking for a 4.1 ratio rear differential to replace it. Anyone have any leads where to find this?thanks

 

[kgb]

normal sound, don´t worry about it.