1990 Chevy Silverado Clattering noise coming from rear diff

The differential lube is not oil. It should be thick and clean - never burnt. Depending on the type of differential you have it may require an additive - but for starters -- the lube is 75w90 or 80w90. This is thick not thin. If you already have chattering there is probably damage done to the ring and pinion gears as well as the spider gears. In order to tell what type of damage is done the differential must be disassembled and inspected closely. If you do not know what to look for, then it is best to have an ASE certified mechanic look at it. If you ignore this chatter, chances are that your rear will bind up and explode - causing even more damage.