banging noise from new water heater

Hello all,
I searched for this answer but did not find it.

About a month ago I installed a brand new out of the box natural gas water heater. It is a Bradford White, 40 gal with power vent. From the start, I have noticed a banging noise that comes from the water heater when it starts up. No water is running and I have yet to confirm if the noise happens before or after the burner lights. It only happens once in a while. It sounds like someone is lightly tapping on the copper water line with a hammer. It goes on for about 7 or 8 taps, then stops. Each time, the taps start out quieter and grow louder, then abruptly stop. Has anyone ever experienced this before? What could be causing this? My previous WH (A.O. Smith) of the same type never did this once that I ever noticed.
Thanks.
Rick

I have it when I run hot water through my pipes.

It's the copper expanding.

My pipe run over I beams, up walls, near HVAC ducts - the works - so it could be a noise against any of these or just the pipes themselves getting bigger with the heat.

It's a tap tap tap.

I'm willing to bet yours might be the same when the heater fires up, as long as we are not talking a BANG BANG BANG here.

I could think of ways to get rid of it (e.g. flexible lines coming off the heater) but wouldn't really bother if a little bit of expansion is all it is. Anyway, whenever I have tried to fix mine, the taps just start up elsewhere!

Insulating the pipe might reduce the noise as well.

the only time insulating the pipes might help (well, it's always good for energy conservation) is if that insulation prevents it from rubbing against something - it will expand and contract based on the water temperature. I get that on my pvc drains as well, but they don't sound metalic, but the same cycling. It could be rubbing against a clamp, a tight hole in a joists, or anywhere else that might be tight. But, I agree with Ian, that's probably what it is. It could also be the exhaust vent. This gets hot quickly, and is subject to the same problems...it expands and contracts as it gets hot and then cools off. If it is rubbing on anything (including it's own joints!), it can make some noises.

If the pipe happens to be rubbing against something hard, you could wear a hole in it over time.

I did a little more digging into this issue last night. The pipes are all solid, and don't clang. I wiggled them all around the heater and no clangs at all. The pipes are also insulated, as I have a hot water recirc loop. I did notice something strange though. If I grab the top of the water heater and attempt to shake it, I hear the noise I was describing coming from inside the tank. It sounds like a metal flapper or something coming from inside, or just below the blower assembly. This explains why I hear the noise without running water, as soon as the blower fires up. The parts that sound like they are hitting one another do not "sound" like they are in the water, but in the vent running up the center of the water heater.

On other curious thing. When I first fired it up, I had to wait a couple days before I could finish the venting. I did not glue it together because I had to take it apart to change a couple things with the vent outside. Anyway, when I did go to finish up the vent, I had to take it apart, and I noticed a black powdery substance inside the vent. It had the consistency of baby powder, and was not a black coating, but a loose powder. At the time, I thought it might be a protective coating burning off because it was new, but now I'm wondering if it is an air-fuel mixture issue, and if this banging noise is related.

Thanks for any advise you may be able to provide.

There could be an air damper in the flue system, and if there's a wind, it might bang around a little.

That's what it sounds like, but does Bradford White use a metal flapper? I would think a nylon flapper would be a lot quieter. Also, it doesn't happen when the wind blows, but when the blower starts. I would think they would have that fixed too. Makes me wonder if something is loose. Also, I'm wondering if this black powder has something to do with the air/fuel mixture, and if these issues are related.

I had a similar problem with a ao smith condensing unit, the water pressure in the house was fluctuating. when the pressure rose it would start clicking. We installed a Pressure reducing valve to stabilize the water pressure and the clicking went away. Don't know about the powder??

It's not a water pressure problem. It is definitely happening before the burner lights, after the blower starts. This happens even if there is no water turned on anywhere in the house.