[F30320dnz]

I have a 2012 F30 320d.
On take off the front seat belts tension a bit tighter like most bmws of the era.
My drivers one occasionally makes a noise like its skipping or slipping as it tries to tighten,
Sometimes it is fine, about half/half for working fine or the noise.
Has anyone struck this? Is there any fix or what do I need to replace?
Also guessing you cannot swap the left and right belts with each other?

Cheers