So i have an A4 B6, and ran into quite a problem yesterday...My car has an awesome purr ever since I straight piped it...but I was driving yesterday and all the sudden the thing is SCREAMING and sounds like someone put a deck of cards in my exhaust. I didn't know if there's a possibility of blowing out my flexpipe from back pressure in the straight pipes? Help

Here's the upgrades I have so far:
AFE Intake
Custom bent straight pipes (no mufflers, no resonator)
Boost Control
Eiback lowering springs and struts