HVAC experts are of the opinion that these kinds of noises might occur when a thin sheet metal of the duct work expands to accommodate the change that has taken place after the cleaning of the ducts. This happens because the unit functions at its peak efficiency. There is nothing clinging to the walls of the ducts which in turn would cause a hindrance to the air flow. Hence, when you hear such a popping noise there is not much reason to worry.
At the same time, one must be aware of the fact that banging and popping sound might also occur when the ducts are not cleaned properly.
There are some ducts which are prone to more noise making than the others so even the size, shape, and thickness of the ducts in your home play an important role in creating those uncomfortable noises from the ducts. This is because duct works are found in three shapes square, round, and rectangle and all these three shapes do not handle air pressure in the same way. For example, the sound shaped ducts can handle more air pressure followed by the square and the rectangle ducts. The maximum popping sound comes from the longest side of the rectangle.
Similarly, thickness of the duct work is an important criterion for the noise. For example, ducts which are thicker in diameter are less prone to noise than the thinner ones.
Another important factor that ought to be kept in mind is that a creased duct work might cause lesser noise than the ones which are not creased.