It sounds similar to what I'm experiencing at the moment with mine, apart from mine sounds awful when first starting up too, the engine is running sweet so I had mine at a custom exhaust garage in my town yesterday and up on the ramps, turns out I have a rusted hole at the rear, and the cat has gone at the front, its the cat that makes the worst noise though under load, my fuel is terrible too, and apparently the fuel consumption is crap when the exhaust is shot, especially if the cat has had it too.