I wouldn't be too concerned.
My car has had this issue from basically day one.
It does not happen all the time, but I have experienced it since I got the car.
I had the DCT update (31.1) installed and it still did it. I had update 31.2 installed, and it still does it.
I am not overly concerned about it, but will continue to report it. Problem is, it is intermitent and follows no pattern that I can discern. So, it is difficult to have a tech replicate.
As far as frequency, I would say it happens here and there. Maybe in 2-3% of all downshifts...that is just a shot in the dark.....but prolly pretty close.
If you figure out the cause, please share. I will do the same.
How frequently does yours have the intake backfire?
edit: Seems like my car has, or has had, nearly all the issues people are reporting. Update 31.2 solved MOST of them. I think as they fine-tune the software, it will only get better. I plan to continue to report this issue until it gets sorted out.
Tim (apparently likes "3" cars)
E90 M3 -- Current ride
2004 GT3 -- Sold
1997 M3/4 -- Sold
1995 M3 -- Sold