The oversize conversion plate theory which has raised its ugly head several times before is a more plausible reason for the clonking noise. It should be easy to check the splines interface though by removing all grease from wheel and hub and testing the fit by feel. On trads this only caused issues if the spinner or hex nut was not tightened sufficiently against the hub taper.
No doubt the recommended grease "cure" is a short term botch.
If it is the conversion plate oversize holes causing the issue I would be tempted to suggest taper nuts with matching machining of plate.
