This weekend I started my '95 PSD and headed on to the highway. About a mile down the road I went to pass another car. I stepped down on the pedal, the auto trans stepped down, the turbo wound-up and POP! This popping noise started! I pulled over opened the hood and I could here this popping noise comming from the intake. Definetely a non metallic sounding pop-pop-pop. Blown head gasket? Todd

 

probably poped a gloplug, might of wiggeled loose or blew the middle out of it. does it sound something like an engine running with one spark olug missing, remember the gloplug is under valve cover. spark plugs are not,

 

yeah maybe that(glowplug) or something on your intake has come loose, or you blew a y pipe boot or bent some pushrods.
start checking stuff.....