Has anybody else had problems with new cats on A3s?....

Recently stuck a new one on, and it makes a really annoying noise; similar to the sort of noise you'd expect from a blowing manifold/ downpipe gaskett, but it's coming from where the two pipes meet on the cat...

It mainly does it up hills in second gear, and it's driving me mad! If anybody knows if this will just go away with age or whether it's just a sh*t cat i'd greatly appreciate it......

Cheers guys....