The ratcheting sound is the safety clutch activating because the burrs are jamming/ed. This can also happen if grinding hard beans (happened to me with some under roasted beans), but it only activated occasionally for a second or two.
Are you sure you dont have anything stuck anywhere preventing movement of the lower burr through its entire range. Have you taken out the burrs and given everything a good clean?
The other possibility is that there two nipples on the lower burr mounting plate, that correspond to two holes in the bottom part of the lower burr (so the shaft drives the burr through these pins). Perhaps when the burr was installed it was sitting on top of these pins and not on the lower plate? and when it jammed it turned and fell down onto the pins? This would be a major change in height of the burr though (3mm or so) and I would think if this occurred you would not be grinding anywhere near fine enough at all. Worth a closer look though, as it does sound strange.