On my old X3 M40i (which cracked, popped and burbled better than my old M4 ZCP) I noticed the exhaust noises sounded better as the exhaust warmed up. If you drive the car really hard for a few minutes and get the exhaust hot, the exhaust noises are much louder. The exhaust noises are random as well (each time is unique), and it seems the better of the noises only appear when driving the car hard.
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