I recently decided to try and determine what was making this noise in my 99 grand prix gt (3.6). It is either a clicking noise or some type of exhaust leak I think but I cant figure it out. I replaced the flange gasket thinking my exhaust was leaking but that didnt make a difference. Then I started the car and went underneath and it sounds like its just to the side of where the flange gasket goes, to the right of it if you were in the car. So heres the video, you can hear it every time I accelerate.

‪Grand prix noise‬‏ - YouTube