How can I fix the noise caused by this hot water heating pipe and my wall?

Question

My house has hot water baseboard heat. It's generally pretty quiet, except for a loud, repetitive ticking/popping sound when it first turns on. From what I've read, that usually means there's air in the pipes, but in this case I don't think that's true.

There are three reasons to think otherwise. One is that it's a closed system with a (working) air separator. Two is that the noise comes specifically from one area, and can be temporarily stopped by pushing on the wall. There's no easy way for me to share video here, and there's not much to see anyway, but I did record a short audio clip that can be downloaded here. You'll hear the noise for about 9 seconds, and then you'll hear me pressing on the wall (not very hard, by the way) which creates a creaking sound, then the sound stops (at least for a couple minutes).

The third reason is that the pipe that delivers (or returns, I'm not sure) hot water to the first floor is visible through the ceiling of the basement. As shown in this picture, it runs half-exposed along the ceiling until it meets the exterior wall of the house:

At that point, it appears that the pipe rests directly on a piece of wood in the circled area. I chipped away a little plaster to get this close-up:

My assumption is that, when the pipe expands, it's rubbing against the wood and making the noise. Pressing on the wall probably moves the wood just enough to affect the sound also (and I can get a similar effect by pressing upward directly on the pipe).

Given this information, my best guess is that I can stop the noise by separating the pipe and the wood a little bit, or by putting something else between them. I can't think of any way to do it without removing a lot of plaster so I can access the area. Even if I did that, I'm not sure what that piece of wood is and if I can shave it down a little. And even if I knew that that was ok, I'm not sure how I'd shave it down, given that the pipe is pressed up against it (when I push up on the pipe, it doesn't move visibly). Since the two are in contact and that gas pipe runs in front of it, it won't be terribly easy to get hands or tools in there.

So, my questions are:

1. Does all of this sound like I'm on the right track?
2. If so, how do I fix it by either removing some of the wood or squeezing something between the pipe and the wood?
3. If this doesn't sound right, what else might I be missing?

Answer 1

Yes, thermal expansion of pipes against supports is a common reason for noise, and that certainly could be the place it's coming from, though it could also be in the concealed space if you have not isolated it to that location by listening.

Might start with some spray teflon dry lube, (commonly available from bike shops, among other places) as least effort ripping things apart that might help. Ideally spray when the pipe is cold and spray when the pipe is hot, as well. Pressing up on the pipe while spraying may help get it to the critical area.

Answer 2

It sounds like thermal expansion to me. Which means that it is rubbing against things and the friction holds it back for moments until it overcomes it and then repeats. Padding will certainly help. Or use brackets (if possible) to isolate it from wood, plaster, concrete etc.