Inspect the bearings, hobbed shaft, idler arm, guide tube, and drive gears. Clean and replace parts as necessary.
Specifically, look for plastic or worn teeth on the hobb, cracks or wear on the idler or guide, or excessive wear or missing teeth on the drive gears.
Reassemble the toolhead as shown observing the following:
Take care to not pinch any wires when installing the new heatsink assemlby.
The fan duct must be installed before the two bottom M3 screws can be installed.
The red ground wire goes thought the heatsink fins and on the bottom right M3 screw.
The fan must be up on the heatsink before the fan duct is installed so that the wires are routed correctly.
The short M3 screw goes in the bottom left hole.
Both heater connectors go with the top of the U shaped crimp facing the triangular latch of the connector.
The heater and thermistor wire pairs are not polarized; the order of the wires can be switched without affecting function.
For more details, see the Assembly OHAI