Exhaust noise usually are'nt hard to isolate leaks.
Does it do it while stationary? If so, try pressing the gas pedal while outside the car and lean head under car.
If's it a leak you should be able to hear the direction it's comming from and you may need to crawl under the car or get it jacked up.
Is it a standard exhaust? Has it just started happening?
Furious is correct in saying that as metal heats up it expands and if it's loosing power that could well be the cause.
I don't know much about cat convertors, but that could be another area to explore.
I had a similar problem many years ago and it took a bit to isolate problem. In my case it was a very small hole around the manifold area. So that's why I suggested to listen to the car while stationary. You might even need another pair of ears to help.