I had exactly this mate, turned out to be a 270 degree crack running all the way around the conical end of the cat, the crack was being forced open on acceleration and making a loud barpppp sound, similar to a blowing exhaust but a bit sharper. Get your car on a ramp with someone in the drivers seat giving it short taps on the fun pedal, if you get underneath it won't take long to work out where it is coming from..