Less than Optimal Sounds from Extruder  

Less than Optimal Sounds from Extruder

In case the embed video option doesn't work: https://forum.prusa3d.com/wp-content/uploads/2021/12/20211222_122837.m4v

I'm having an issue with the extruder. It is making some pretty raunchy sounds right up until the filament makes it to the gears.

I added the MMU upgrade in early November and it was working wonderfully. I had very little trouble with it right out of the gate. The extruder motor was also running silently. At some point, the filament jammed in the hot end and broke off. I removed the print cooling fan, two screws from the heat sink fan, the idler screw, removed the cooling fan mount, then the two top screw from the back side of the extruder motor. I cleared the jam and replaced the PTFE tube inside the heat sink. Upon reassembling the extruder, it was making the noise in the video.

I cannot figure out what is causing the noise. I've taken the extruder apart multiple times since then, and just cannot figure it out.

Here's what I know:

1. If I remove the idler screw and pull the idler door open, the noise stops.
2. The idler gear spins freely. I can flick it with my finger and it'll spin for about a second or so.
3. If I control the extruder manually, the noise happens consistently in the same spot. If I back it up then move it forward, I can repeat the click/grind easily and consistently.
4. With the motor pulled out, if I turn it manually (or initiate an MMU load) the motor spins and doesn't make noise.
5. With the motor pulled out, if I hold the idler door against the motor, I can't tell if it is making (or trying to make) noise.
6. I don't see any sign of grinding on any plastic any where near the gears.
7. The assembly is silent when filament is inserted.
8. I reprinted the idler door, but it didn't fix anything.
9. I verified the gear on the motor shaft hasn't moved and verified that the grub screw is tight.

What the heck is causing this noise? Is it just the teeth meshing together? Do the teeth wear in such a way that changing the orientation of the gears is detrimental? Is it the bearings inside the idler gear? Do I need to add some kind of lubricant to the teeth?

This doesn't seem to affect printing or MMU operation. I've had thousands of filament changes since this noise started. But it is quite aggravating.