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info on the truck. have you done anything to it? my truck makes a rattling noise when I take my foot off the pedal while foing 40 - 70mph, at high speed, high rpms and high boost. I think that it's just the intake cutting out. I would check all the boots on your intake plumbing, from the turbo throught the intercooler to the intake elbow.
I had the same issue to around 35 mph rolling to a stop it would get a grinding/rattling noise. Next day, it was on take off too. Turned out to be my MBRP downpipe was sitting on a crossmember bolt which caused it all. If you have an aftermarket exhaust i would look to see where it is hitting.
Does the fuel shut off upon deceleration (w/ manual transmission)? If I'm at a cruise and lift the throttle slightly, the tone remains basically the same. OTOH, if I lift off the pedal completely, it sounds much different, making me wonder if the computer shuts off fuel altogether, perhaps to aid engine braking or something like that.
I also had that same problem on both my trucks. It was the down pipe rattling against the rivit that holds the firewall insulation on. 4" magnaflow pipe.