It's likely "pinging". Some good reading here:
http://en.wikipedia.org/wiki/Pinging
My Windstar did this when the isolator bolts needed replacement (I never did get the infamous P0174/P0171 codes). In my case, the isolator bolts were allowing just enough air leakage to cause pinging. Did your TSB fix include replacing all the isolator bolts and port seals?
A quick test: Add a can of octane booster to the tank (a typical auto parts store will carry it). If the pinging immediately stops, I suspect you've still got an air leak in the intake somewhere.