it might be when you truck is in a lower gear and making boost and when you hit od the truck goes down in rpm you hear teh turbo realeasing teh air. mine does it all teh time. or when you stomp on teh pedeal and release it right away you will hear teh turbo dump the air.do you feel any power loss. are you on a steep grade in overdrive. maybe your just make boost and that is what you hear is teh turbo spooling up.