I posted about a weird groaning noise coming from the firewall on the passenger side when I first got my runner.
Interesting groan from front end...
If it's the same noise that you are having it turns out that it is the ABS actuator moving, at least that's what I concluded on after doing a lot of research. It actually says in the 4runner manual (I checked myself) that you may hear this noise and it is nothing to be worried about. I thought it was temperature related as well, but it turns out that it does it right after you start the truck, put it in gear, and begin to move. After that it doesn't do it again until you restart.
Just realized someone posted in that thread and I never followed up...