I've been driving my car now almost every day that it's not raining. I feel like I've got the engine tuned really well, everything seems to be working as it should.

Except very rarely I have this one issue. It usually happens when the engine is on the cold side, before it's warmed up fully, under somewhat hard acceleration. I'll be accelerating and as I shift - it either happens when shifting from 2 to 3, or from 3 to 4 - there'll be this whip crack sound. It happens just after the shift as I'm letting off the clutch and getting on the gas. I don't know if it's a backfire because it's so sharp of a sound - it's really like the sharp crack of a whip. Backfires I've heard from carb'd cars tend to be a little deeper sounding.

In the few months since I've got my car running reliably, it's only happened maybe 3 or 4 times. So it definitely doesn't happen often, but when it does it's definitely an alarming noise! Just curious if anyone's run into this before.