New noise - backfire on upshifts

I'm experiencing something new in recent weeks with my lightly modified 135i. On upshifts during hard acceleration, I'm more frequently hearing backfire-like noises - not the burble/popping on deceleration that we all love, but full-on louder single pops. No hesitation during driving, but I've also noticed a slightly rougher idle on cold start.



My car has the 6-speed automatic with Alpina B3 transmission software (so faster upshifts in manual mode).



Is this something a lot of you are seeing, or should I be concerned?