This howling noise from rear end started at about 6,000 to 7,000 miles on the truck. It has progressively gotten worse. I took the dealer Service Manager for a ride in my truck. He heard the noise right away. It happens at highway speeds of 55 to 65mph most often and it is when the transmission is under light load or no load (feathering the throttle) once you apply throttle the sound goes away. Dealer looked into the issue and contacted toyota corporate. They do not know what the issue is but they went ahead and ordered an entire new rear differential assembly. I find this to be problematic because without a satisfactory root cause analysis, they may or may not be resolving the root cause and the new components may be at risk of failing again.