Kitchenaid mixer dropping metal shavings
From the sounds of it, because you were using it for a rather dry dough, it might've had to work more than it usually does, and could have resulted in some grinding of gears.

Now, if the mixer completely bogs down, most Kitchenaid models have a sacrifical gear that's made of plastic that will get destroyed close to the motor (ie, up top, not near the planary action bits near the bowl), but it's possible for it to not be quite bogged down enough for that to get destroyed.

As for servicing them, they're actually pretty easy to work on, provided you keep track of the parts and have a set of lock-ring pliers.  (and if you open up the gear box up top, some grease to re-pack it).

When I helped service a friend's 600, we found plenty of manuals online; but be aware that there's more than one model of 600.  (mine had a different model number than hers).  

I'd personally try to check the planetary gears, and check if there are more shavings in there -- if so, it's a problem and at the very least, you need to empty it out.  If I recall correctly, you don't actually need to open up the main housing -- there's either a pin you can punch out, or a retaining clip at the top of the shaft that you attach tools to, and you can remove the cover without too much difficulty.  (I think it's the newer models that use the retaining clip; see the manual I linked to for those that use a pin)

The other gearbox up above isn't going to throw shavings -- it's packed in grease within a sealed case, so if there are shavings, it's going to not get thrown free (instead, it'll continue damaging the gears)