I believe the noise you're describing was the minor backfire caused by the mixture going very rich on deceleration.

The high vacuum would cause the liquid gasoline in equilibrium on the surfaces of the (wet) intake manifold to vaporize at the same time most air was restricted, giving rise to a momentarially overly rich mixture that wouldn't fire in the cylinder, but ignited in the hot exhaust. The combustion there wasn't very efficient and a this was a source of unburned hydrocarbon pollution.

Many foriegn cars with smaller engines had this characteristic, and an emission control used to suppress it was called a "gulp" valve - it let in some air to restore the mixture upon very high vacuum in the manifold.

Any car with a wet intake will tend to go rich when the vacuum goes suddenly high - on US cars, a dashpot was often used to limit how fast the throttle could close rather than a gulp valve. Cars with direct port fuel injection have dry manifolds and never experience this problem.