Under boost a leaky uppipe would vent, not suck.
Suck...vent... whatever. It'll pop. 
He was getting so pissed off with the car he was ready to part it out. 
Now...here's a question for everyone. If the exhaust cam lobe was going south...could the air/fuel mixture get forced out the intake valve...cuz it can't get out the exhaust...maki ng it pop?
It would depend on what cylinder during the event. More than likely the exhaust heat and pressure would overcome the vacuum with the intake valve open, and to what extent the intake valve is open. Nightmaring back to the carburetor days, if we dealt with a car backfiring (popping) through the carb, (in this case I figure intake plenum?) it was a timing issue (dist. cap included) or a lean misfire. So let's throw a turbo at it and lean becomes a whole other issue, like Earl said: wastegate performance. Just a thought.
