I have had to have a new catalytic convertor fitted to my Citroen Saxo since its replacement it sounds really rough especially when you start up, reverse and over rough surfaces the garage says that the cat has to be coated with soot ie driven for a while to quieten it does this make sense?
A cat won't get coated with soot-it runs too hot but if the rest of the exhaust has also been replaced,that will get quieter,not because of soot but because of it's filling settling into place.
Was this a genuine manufacturer cat or a "cheapo"?;it sounds as tho' the insides(substrate) are falling apart already.It is very unlikely to get quiet!!!
as jc2 says
sounds like a cheap cat,not a good idea on a saxo as it will more than likely fail the emmisions test come mot time unfortunately (have the badge)