Main problems are fuser and the ADU drive. The gears that are above/to the right of the fuser, a blue bushing in the ADU entrance (all gears/axels there, really), the gears on the right hand side of the fuser unit and the fuser drive gears behind the covers around the main drive area seem to squeak.|
Buy some DrySurf and heat resistant grease and you're set. Also, silicone lube works wonders for the gears and shafts around the ADU area.
You can also activate the switches that the right hand door activates to run it open, to see if you can better locate the sound. Since there are alot of different sources for it, it's hard to pinpoint exactly.
The PDF that JSC linked is excellent.