Noise when engine gets hot

[Silverado0075]

Silverado with 5.3 V8 and 8 speed transmission. On hot days when I do a lot of in town driving, I hear what sounds like what could be a fan running to cool the engine.  The weird thing is, if I put the truck in reverse, the sound stops.  I hear it in park, neutral, and drive, but it stops in reverse. This sound only happens on hot days with stop and go driving at low speeds. It's the fact that the sounds stops in reverse that I can't figure out. Any input is appreciated.