Best to look into popping noise in chimney right away

While I don't have a problem with a noisy water pipe, I do have a problem with a noisy chimney.

Some years ago, I replaced my old brick fireplace with a firebox insert and metal flue.

All was well until a few years ago when the fireplace started making popping noises after warming up.

Metal expansion is a good guess, but based on the age of the installation and the fact that the noise just recently started, we think it more likely that either the orifices in the gas insert are plugged or the chimney is extremely dirty.

Expansion and contraction increases and decreases depending on the temperature of the ambient air around the metal.