Ford: ford crown vic 93, ford crown vic, dual flowmasters

Question
hello, let me start by saying that i recently replaced the engine in my 93 crown vic,everything seemed fine i then replaced my exhaust with dual flowmasters, i then could hear a vacuum leak, i found it and replaced the hose, the car dropped down in idle, and sounded good, but now, when you first romp on the gas,sometimes there is a very loud metallic sounding knock, coming from somewhere near the manifold it sounds like, almost like a backfiring bang but it only happens if you hit the gas kinda hard if you were going to romp on it, but then it sounds fine throughout acceleration what could this disturbing sound be??

Answer
CHRISTIAN

on the back of the motor there is a pollition canister that i would bet pulled off.its hard to see you have to feel for it.on your exhaust on the passenger side there is a small metal pipe that goes to the back of the motor then to hard rubber thats where it comes apart.the only other thing i can think of is the o2 senser is going bad. hope this helps.

ken