This sound has me puzzled! I recently bought a 07 1500 RWD w/ 88k miles. When the truck is running there is a hissing/squealing sound. the sound will fluctuate a little bit but it is always going. The A/C does not affect it. In fact, when I turn the key to the run position it starts, but it gets louder when I turn the engine on. It is coming from the mid/lower passenger side. I cannot hear it from the engine bay.The "dealer" replaced the heater core, evaporator, compressor, and they did a blend door repair. They are claiming the A/C system is 100 percent vacuumed. Any ideas? Help would be greatly appreciated! Otherwise the truck runs great.