edit:VGTs have a tendancy to make weird sounds.. they also function as a sort of 'engine/exhaust brake', as well.6.0L Turbocharger
The 6.0L Power Stroke is equipped with a variable geometry turbocharger (VGT). The 6.0L turbocharger is very efficient & provides quick spooling with plenty of airflow through peak RPM. However, the vanes of the VGT that control the displacement of the turbo are prone to sticking from soot build up. If the vanes stick, performance will be sacrificed.