Goldeneye

Hi All, I am hoping someone might be able to help me understand an issue. I bought a 2016 A7 petrol a couple of weeks ago. With only 19,000 miles, the car is stunning and initially I was thrilled. However, a couple of days after buying the car I started to notice what I can only describe as a bad egg smell coming through the vents. This is particularly noticeable when starting the engine from cold and then reversing and setting off. It then seems to be okay when driving, there is perhaps a very faint smell that comes and goes but it really is just when starting off. I have got the car booked in for a diagnostics check but that is still a couple of weeks away and so in the meantime I guess I just want to make sure that it is safe for me to drive. I immediately thought it could be one of the two cats but there isn’t a smell outside of the car or any lights coming up on the dash. I have read that it could be the battery or transmission fluid possibly. I have had a lot of cars in my time but never with a problem like this and so any guidance from the forum members would be greatly appreciated! Thanks David