Engine Performance problem
2009 Mini Cooper S 4 cyl Front Wheel Drive Manual 15000 miles
G'day. I have the Mini Cooper S turbo manual. I'm experiencing loud backfires on upshifts (mainly thru 1st --> 2nd and 2nd --> 3rd) and downshifts (mainly 5th --> 4th and 4th --> 3rd) and also under engine-braking. These are pretty big bangs - much louder and sharper than the expected happy, throaty burble - they're loud enough to startle passers by and sound like a large firecracker.
The sound seems to come from the exhausts (which are quite sooty).
BMW have checked the computer report and found no recorded errors but the thing goes on.
It happens many times each day, might be slightly worse on hot days and seems worse when using 98 RON petrol instead of 95.