It's difficult to tell what might be causing the knocking sound without actually hearing it.
The only moving part in that area is the wheel bearing itself, but I wouldn't expect it to make a knocking noise like the one you have described.
The only other thing I can suggest is to check the tightness of the Allen bolts that hold the side cover in place, just in case the cover is moving slightly and making the noise.
If you can upload a short video to YouTube and then post a link here, it might be easier for us to diagnose the noise once we can hear it.
Alan