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Can you hear this noise from under the hood when the hood is open and engine is off?
For the brake pedal to go to the floor and the brakes not apply that has to be air trapped in the system. The master cylinder, booster, nor ABS could cause that as an intermittent issue in most cases, though a trapped air pocket could. IN this order, I would try a brake bleed at each wheel, then if it continues, do a new master cylinder.
This noise does it seem to be coming from the turbo, if so are you running a power programmer on the truck?
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