gear backlash? can someone explain the rattling? i mean, when you hear rattling, in a component like this, metal is most likely hitting metal. that doesn't sound good.

 

Here's a technical definition...

Gear Backlash: Properly functioning mechanical systems need to have a certain “clearance” (“gap”, “play”) between the components transmitting motion under load. Clearance is necessary to avoid interference, wear, and excessive heat generation, ensure proper lubrication, compensate for manufacturing tolerances etc. Clearance in the gear mesh means that the gap between the teeth of one gear is by a small amount larger than the tooth width of the mating gear.
We also find a certain clearance in the rolling bearings, namely a small clearance between the inner race, rolling body (ball, roller) and outer race of the bearing.