Hello,
There are two likely possibilities for this leak.
The first is the pinion seal located at the front of the differential, where the pinion emerges and attaches to a flange, which in turn attaches to the drive shaft. If this seal is leaking then you should see a trail of oil from right underneath the front face of the differential to the point where the oil drops finally break free.
The second is the oil breather tube that sticks out vertically from the top edge of the joint between the left axle tube and the differential. This typically overflows if the oil has been over-filled (or if you recently hung your car upside-down).
There is a small felt pad inside the breather that acts as a filter or damper. This can get clogged over time, and the resulting high pressures that build inside the differential eventually cause the pinion seal (the next weakest link) to fail. So you might be looking at multiple things.
Also, if the oil is clean then it might be difficult to trace the path.
The knock sound can have many sources (also unrelated to the differential), so can you narrow down the source? (front/rear/left/right)
Possibilities include motor mounts, transmission mounts, subframe mounts, idler arm bushing, differential upper mount, muffler 'donut' mounts etc, etc.