My truck has developed a new problem. It sounds like it does when I am pulling our camper trailer up the mountain and the engine/transmission starts to get hot. I am guessing that the transmission cooler fan is kicking on. The result is that the engine sounds a little louder and sounds like it is really revving up. It now does that all the time without pulling a trailer. Occasionally, the truck does it right out of the garage in the summer when I have been using the air conditioner. I do shut the a/c off before I shut the engine off, but sometimes the truck over-revs until I get into high gear and get going down the highway. I don't know what the deal is and I haven't been able to find anything on the internet either.