I have a 2006 auto g35 coupe, I bought it bout a month ago came with a muffler delete with I believe a performance muffler or resonator had a nice aggressive VQ sound until about a week and a half ago I was going to the store on a arizona night but it was bout 110° outside, I had pressed the gas then the brake then it seemed as the car got stuck in gear, so I proceed to park at Walmart, when I turned the car on again i immediately noticed my exhaust sound wasn't the same at first i thought it was because it was so hot outside but i let the car cool over night but it still sounds horrible when accelerating, I've tried to see if it's a exhaust leak but found no leaks. I think it might be the cats but I have no clue.

 

I had two bolts on the y pipe loosen up on me after a few months, not sure why - you couldn't tell by looking, but it had a horrible leak. I immediately noticed the sound difference. You may want to put it on a lift and check all the bolts are tight, I know its a pain - but would at least confirm if its the cats or not. You may not see a gap, but if its not fully tightened, you will get a leak. Hope that helps...

 

I second that, if it sounds like trash it's probably a big exhaust leak.

Check the transmission fluid level, perform a drain and fill if it hasn't been done in the past 30k miles, there is probably something wrong with your transmission and it went into limp mode which is unrelated to an exhaust leak.