I don't think you have a muffler problem.Like I said in post #6, it is a combination of both, header and test pipes. Headers more than the TP. I have done this before, so I speak from experience.

Get yourself a large resonator and it will solve the problem. Looks like so:

http://www.mikekemper.com/elantra/exhaust/resonator.jpg

These can be installed so that the fins are facing the exhaust or the other way around. If the fins (openings) are facing the exhaust is will make it more quiet.

 

Went to an exhaust shop today and their performance guy told me its likely both causing the sound, if I want it to go away get rid of the HFC and put my stock ones back on and it should get rid of the rasp. Now he's and exhaust specialist but I don't think he's done much G35's he told me these HFC don't make much of an increase in power at all infact if I were to leave the HFC on and he put a big resonator on the midpipe it would be just the same (for power increase) to just take them off and put the stock cats back on merely because resonators restrict flow, he mentioned HFC and performance mufflers are a very small power increase 3- 5 HP unless your engine is heavily modded.
Any input and or thoughts on this is appreciated, now if any of you are going to say "He's full of it they do give a decent power increase" than please back up your statement with proof like a before and after Dyno pull.....NO but dyno's and no "but the berks website said 15HP increase"......pff please.