assuming you have a stock exhaust, One with the twin head on it???
Have a look down the righthand side and you should see a flap over the hole, blocking it..
Like this...
I believe the cable you can see in the next photo opens the hole when the pressure builds up or the car reaches a certain rpm!! This could be the cause of your noise! I find mine gets louder around the same time!
There is a cable on the upper right hand side that opens this flap!
Just a thought!!!!