Recently my 2008 GMC duramax has suffered a devestating loss of power, accompanied with a sound inside the cab that sounds like air coming through the vents. Upon heavier acceleration, the air sound gets louder. The truck rarely smoked but it has started smoking excessively. Cleaned the air filter, inspected the entire intake, inspected the turbo hoses, found nothing. Any thoughts? Oh, and the gas mileage has dropped off the charts as well.