Differential

[Brendanmck]

Having a bit of bother with my csl getting a rotational noise from the rear left it gets louder with speed. My mechanic has checked everything at the rear and seems to think it is one of two things either a drive shaft or the diff . First up. Are Both drive shafts the same and could we swap the left one to the right and vice versa to see does the noise shift to the right ? And secondly if the issue was with the diff and it needed replacing is the diff on a csl the same as the standard m3? Just wanna consider a second hand replacement . Last thing is the oil inside the diff worth changing also before I would consider changing a diff?