W261w261

My son's 2014 A4 produced a backfire-like noise upon a restart (20 minutes), which he said came from the "back of the car." It's a 2.0 direct injection motor, and I'm wondering what could have caused this. The only thing I can think of would be some unburned gas in the cat, and when the heater came on it somehow lit it off.

I know this is the B6 forum, but it's the one I'm in so thought I'd give it a shot. Thx