I think I have the same problem, the knocking sounds identical only under slightly different conditions. Mine happens whenever the left rear is under load, not only under acceleration but also when turning right.
I have found that it doesn't only happen whilst there is power through the drivetrain but just turning right at a reasonable speed with the clutch in.
I'm now starting to think it could be the outer cv joint of the left rear drive shaft as I'm assuming the angle of the joint will change under load regardless of weather or not any torque is being applied. What do you think?
Hope i'm not hyjacking your thread only trying to open up discussion of possible causes.
Will try to get a recording today to confirm same noise and have a look under the car to see the cv boot is in good nick. I'll also check the spare tyre well for any cracks.