When gears shift around 1600 revs in my 2014 diesel V6, there is a slight hissy "woosh-woosh" sound that isn't there otherwise. What could it be, any ideas? Anything serious? I am on a trip that's supposed to take another 6 weeks and several thousand miles. Mostly asphalt, but still a bit concerned. Gearbox oil was replaced last year, engine oil a couple of months ago.