From the conditions that you describe this "fluttering" occurring and the fact that it stops with throttle input my guess is you are feeling vibration from the AFM kicking in. There are several threads on this issue if you do a quick search. It is considered normal, but if it bothers you there are updated motor mounts that will greatly reduce the vibration you feel.My g8 has a flutter that you can feel in the steering wheel and the seat of your pants. It happens when the car is not using much power and a slight push or let off of the gas pedal will make it go way. The dealership says it is the exhaust and there is nothing I or they can do. Any one with a fix would be greatly appreciated. Thanks
You need to make a correction to the GT muffler. There is no flow in the third chamber. All the flow comes in the perforated inlet tube. some exhaust is absorbed in the first chamber. The rest is forced to flow out of the perforations is the second chamber.The tube dead ends at the third chamber. Sound is further absorbed by the through the walls of the perforated chambers 1 & 3. Flow then goes out the "J" tube to the outlet.
The GXP is a typical 3 tube turbo style muffler. The difference is the dual outlets. the GXP only has fiberglass packed in the centre chamber. Not the ends like the GT.
This dynomax turbo is the same design. Just swap the centre and outlet tubes. It is great to actually see the guts of the GXP muffler. Kind of what I expected it to be from the sound. I went with the Stainless Works Turbo chambered mufflers.