Hi,
I also have a 1.2 VTi and there is a slight rumble during idle when the aircon is on. I have been assured that this is normal by the dealership. Personally, I love the louder exhaust as it sounds like this little engine wants to prove itself.
The car should be under warranty and all you need to do is request an inspection form from the dealer clearly stating what the query was and if a fault was identified or not.
Keep that inspection form safely as this is proof in writing that the vehicle is fine. Should anything occur regarding the idle or any problems you have, you have it in writing that the car was taken in for an inspection for the same problem at a much earlier date before something major could have occurred.
If the rubble persists even with the aircon off, I suggest you take it in.
The loud exhaust makes the car sound more masculine therefore I couldn't name my Citroen "Ella"...