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*Source Wikipedia: Exhaust system backfires occur in engines that have an emission system malfunction, like an air injection system diverter valve problem, or an exhaust leak. In some high-performance vehicles, when a driver shifts up and lets off the accelerator, the engine has a moment of running rich. This causes an incomplete burn which causes the fumes to explode in the exhaust system along with an audible pop or bang sound. This is a result of working equipment, and is unlikely to cause damage. Also, poor or unregulated engine timing is often a cause of intake backfires, but can also be responsible for exhaust backfires. Improper wiring in the ignition can also lead to timing issues and backfires. Low fuel pressure, clogged fuel filters, and weak fuel pumps could cause a severe lean air-to-fuel ratio during fuel injection. Hope this helps.

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