I drive a 06 Subaru Outback (AT, NA). I've had it for about two years. There is a sound which seems like its coming from the bottom of the car. It sounds kind of like the engine revving but when I let off the gas the sounds comes down slowly so I don't think it's the engine but something under the car. A friend told me it might need new differential or transmission fluid. What could it be? Just thought I'd get input here before I take it to the shop later this week. The sound has gotten louder and louder. Thanks.