NEW: Current SpinOlution wheel models have a band on top of the treadle hub to keep it closed as a permanent fix.
The knocking is happening because the oval treadle is opening up and hitting the bearings instead of staying closed around the bearings. This could be caused in a change of humidity causing the wood to shrink and the treadle to loosen and open.
Temporary Fix: Put a rubber band around the outside of the treadle to keep it from opening. Just in case you are traveling with your wheel to a new environment (especially a dry climate) it would be a good idea to bring a rubber band with you just in case the wood of your wheel is effected by that new climate.
Permanent Fix: Add 3 layers of painters tape to the inside of the treadle (so that it is hidden), and gently close the treadle to secure. This should resolve the knocking.
If you are still having knocking issues, email photos of your wheel (including treadle and any information regarding how you've tried to resolve the issue) to info@spinolution.com and we will help you troubleshoot.