The decel pop is a sure sign your engine is running too lean and you'll be losing power. Opening exhausts pumps more air. If you dont add more fuel you dont get combustion until it gets to the exhaust where it ignites due to heat it then backfires and pops.
Hideous sound as its the sound of unesessary power loss. Especially as R3s are so easy to access with Tune ECU to sortthis.
Simply add more fuel on the L Tables in Tune ECU or there are plenty of good maps.