This article is from the Toyota Tundra FAQ, by Steve Yee with numerous contributions by others.
This is known as an ABS Clunk. It originates from the right side
engine compartment of the Tundra. The ABS system will do a self test
during the first batch of forward or rearward movement that your
Tundra has within the first 1-2 minutes of operation. This is normal
due to the ABS self test and will occur when your Tundra is either in
a forward or reverse gear.
 
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