Does the noise only occur while the motor is working hard, or does it happen while pedalling and free-wheeling too?
Is it a regular knocking noise relative to the wheel rpm, or just an intermittent knock?
Does it get louder when you brake?
Is the wheel smooth to turn by hand or can you feel any tight spots?
Can you video the noise and upload it to YouTube so we can see and hear it?
It might also be a good idea to check that your axle nuts are still nice and tight and have not worked loose.
Please give us as much information as you can, and a video would be very helpful if you can manage it.
Alan