Q: I read an article by you guys on water hammer and it prompted this note. 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. We burn wood but use gas jets to get the fire started. All was well until a few years ago when the fireplace started making popping noises after warming up. As time has gone on, it has gotten worse.

My guess is that the metal flue pipe is expanding and contracting but I honestly have no idea what is happening or how to fix it. Do you have any suggestions?

A: 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.

We suggest you look into this right away because if left alone this could develop into a dangerous situation.

If the gas feed is faulty, this is not a do-it-yourself project. We recommend you give PG&E a call and ask them to send a technician out to check out the gas feed.

The technician will be able to tell you if the popping is from gas and how to fix it. It may be a simple thing you can do yourself or he or she may clean it up for you. But it's also possible that that gas feed is shot, in which case you may need a plumber to install a new one.

This advice also pertains to anyone who has a non-wood burning gas fireplace.

Another likely possibility is that the chimney is dirty. When was the last time you had it cleaned? Creosote buildup can pop as it heats up in a dirty chimney. The popping itself is not so bad, but it could be a precursor to a life-threatening chimney fire.

On the other hand, if the popping you hear is just the sound of metal moving, this is normal. Metal expands when heated and contracts when cooling. Expansion and contraction increases and decreases depending on the temperature of the ambient air around the metal.

We suggest you have the chimney inspected and swept by a certified chimney sweep to insure the chimney is in good repair and clean. With any luck, a good cleaning will take care of the problem.

The Burnett brothers are freelance writers. E-mail them at sweatequity@sfchronicle.com.