The initial rough idle and exhaust burbling for a few seconds after cold start is the CAT heating phase. It is a separate cold start routine in the programming to bring the CAT up to operating temperature as quickly as possible. The colder it is, the more noticeable and for a slightly longer period. Don't worry about it. You should hear a V10 R8 or a Lambo after cold start. Sounds like a top fuel dragster idling for 10 or 15 seconds....