Our mufflers are pretty poorly/cheaply designed, and because of that, they get very hot under any kind of load.
During normal operation at idle, they are about 215*F, and after driving up a moderate hill, I measured my mufflers temp at 320*F! Even if your driveshaft isn't flinging grease on your exhaust, you muffler will get so hot during any kind of load, it will tick, snap, crackle, and pop! The sound will be most apparent from the rear pass side wheel well. Just drive up a couple of floors in a parking garage, park, and check your rear wheel well and you will hear and probley smell it.
Check out my post below for more details:
Burning rubber smell right rear fender well.
After 30k miles of this, I have given up and accepted this as normal for our 4Runners.