My gas furnace motor has become very noisy something is rattling inside. I have removed the casing to the fan and squirrel cage is able to turn on the shaft. There is a locking ring with a small bolt but tightening this only locks it to the shaft, the cage is still able to turn on the shaft. I think this is what's causing the noise. How do I fix it

The squirrel cage usually is slipped onto the shaft and then a allen or hex head bolt is tightened to lock it to the shaft. Also from what you said the cage itself might have broken loose from its mounting that goes on to the shaft. If this is the case you will have to replace the cage/fan blade part of the unit.