I have a Mazda 6 5sp Manual, When I stop or idle I hear loud crackling and popping (almost like popcorn). It only makes the noise after the car has been running at high rpm (like freeway driving) and then come to a stop or idle (at stop lights). No noise or rattling can be heard when I start it or when its running cold. The noise goes away as soon as I start to accelerate again and returns when stopping. Ive checked all the heat shielding, nothing is loose. The noise is comming from the back part of the engine near the fire wall (but not from inside the car). Ive taken it to the dealership and they cannot pinpoint the problem. Any ideas? Im at a loss here and it seems to get louder every day. Im positive the sound is not comming from the motor itself.

 

I had a similar issue once on a different car. Check the Cat. get under the car and hit it a little with a rubber mallet. If you hear something rattling around int here you need to replace the Cat, or do what i did gut it out and stick a straight pipe through it. =]