At any time, day or night, whether engine is cold or warmed to operational temperature, whenever the truck is moving, this noise occurs. Its not always constant, but rather mostly broken into inconsistent intervals of serveral seconds. When I touch the brake pedal (whether or not any actual braking or slowing), the noise abates for several seconds, then continues as described. So my mind leads me to vacuum booster on the power-assist braking assembly, or perhaps the turbo blower. I visually checked, but all appears normal. Odometer reads 155k miles. Serpentine belt appears in good shape. Performance is normal, nothing indicates a problem, except this noise.