Hi,
It might be in the tow/haul mode, it downshifts when you are leaving the freeway and not on the gas to help save the brakes.
You might also hear the fan clutch kick in, it does sound similar to the engine reving way up in RPM's. Many times when I leave the freeway in my motorhome, if I get stuck at a light, the heat from the engine will build up in the radiator, and the clutch will kick in for the first 15 seconds of driving.
When the clutch kicks in, then the fan is run 1:1 with the water pump speed. Normally the fan is idling, so it might turn 1/4 to 1/3 the speed of the water pump that it is mounted to. It makes much more noise when engaged.
Have you washed the radiator lately? You might have a layer of dirt that is preventing the heat exchange. Wash with a garden hose, and see what comes off it.
Fred.