The stupid chirping sound at low revs - higher throttle. After fixing all exhaust leaks, I thought the inside of the cat was broken, thus the chirping, but it remained after the change. Might be the straight through design of the Invidia, or busted exhaust manifold gasket, no idea. At least I started seeing steam etc out the back, which means the old cat was in pretty bad shape, I might have been in trouble at the next MOT.
Why do you want to get yours replaced?