My printer is making a "rattle sound" (I'm posting a movie, You can hear it in the background) and it starting to drive me crazy because I can't locate it source so maybe someone can help my.
There is nothing wrong with print quality, also when I move the extruder by hand there is no noise, everything moves smoothly.
Hard to tell from a video, does sound noisy though.
Have you considered just going through all the pulley/stepper motor grub screws and checking them for tightness, as well as greasing all your linear rods?
Checking your belts for tension might be a good idea too.
If as you say, the extruder moves smoothly by hand and there's no issue with print quality, I'd be inclined to just use it and try to ignore the noise.
I've done all the maintenance procedures and everything is tight and greased. I've done a lot of long prints with this noise but I'm always worried when I have to leave the printer for night. Guess I will have to deal with bearings replacement and see if that helps.
One last thing I'd try before throwing the baby out with the bathwater.
If you are going to pull out the central rods/extruder, try sliding the twin 6mm rods out of the extruder assy, squeeze quite a bit of grease (strongly recommend Superlube teflon grease) into the linear bearing, put your thumb over the other end and push the rod back in, pressurizing the grease in the linear bearing (if that makes sense).
This will really inject the grease into the little bearing races inside the linear bearing much better than just wiping it onto the rod, but you need to be very careful to gently slide the rod in (giggity) so as to not lose the tiny little bearings inside.
Excess grease will pop out from under your thumb, and can be wiped away.
I have only done this with other printers, not the Z, by the way.
Another trick I've used is to leave one of the tiny grub screws on each of those twin rod axis's slightly loose (which is only 2 out of the 8), which does allow the axis's to move much more freely and has had a very positive effect on my machines.
I would avoid playing with the external 8mm rods and bearings. It's unlikely there's an issue there, and it's major ops to pull that all out.
It seems that I have solved my issue (with help from support) so I though I'm gonna share the solution - I have loosen screws that holds bearings of X and Y axis, let them set and tighten them back, just like shown here: