Drivetrain Whirring / Knocking Noise

So over the past few days I've noticed a whirring / low fast knocking (is best I can describe) It's more prominent after a few miles driving - so maybe once warmed up, it's coming from the front left.



 


 So I jacked up the car and had a look thinking it might be a wheel bearing, wheel is solid in all directions. Rotates freely. However I did notice when I spin it fast in reverse I can hear a knock, it appears to be coming from the drive flange coming out of the differential.

 

I noticed the constant velocity joint jumps slightly each rotation, but I can't replicate spinning the wheel forward. 

Here is a video of it, if you look closely you can see the jump and hear the knock.


 

Do you think it's the differential, a drive flange tooth, a bearing??


 

Any ideas before I start stripping of advice would be greatly appreciated!


Video
MK1 Golf GTI Drive Noise - YouTube