your midbox and muffler is still the stock one?

This is a common tactic by exhaust shops, they will change extractor first so that the annoying sound will appear. Then after a few days/weeks when you tak tahan the sound, then you will go back and change muffler. After changing muffler, you will then experience some "bunyi berdengung" from the middle of the car and then you're forced to change the center bullet also.