What causes the "pop" and "burble" noises on overrun?

Question

I love the sound, just wondering what causes it.

Answer 1

One cause of pops is unburnt fuel hitting the exhaust manifold - many engines use fuel to cool the cylinder when you lift off, so this sprayed excess then burns as it hits hot metal in the exhaust.

The burble on overrun is usually caused by ignition timing being retarded.

Aside from those, any valve or gasket leaks could also cause pops.