Don't worry about it, you got a decat and with the colder weather the ecu is probably throwing more fueling in and enjoying the denser air!
Mine does it now and again but I put that down to the engine being cam'd but out on track after 15mins it will back fire most of the time when i lift off or change gear.... noticed that around castle combe where I sit high in the gears all the time!
Load of my mates got 106 Rallye's or XSi's with 16v lumps and they all do it! No problems with the enigne or exhaust, one lad had his for 3 years now.