Old Hot Water Pipe Making Noise (Why Now?)

House has ~25’ length of straight* copper pipe (hot water delivery, not sure if it is ½ and ¾â€, not insulated) that runs from a guest bathroom to the kitchen sink. Earlier this winter, I started to hear sounds coming from under the house, when the water coming out of the kitchen sink’s hot water faucet finally heats up. It hasn’t done this in years past, and nothing has changed recently plumbing wise. The sound is like a single “pongâ€, not really like a water hammer.

So I assume the pipe hitting something now when it expands. If there were a tight spot, I would think it would’ve been making the sound all along. There is no noise as the pipe cools down, also no sound when just running hot water to the bathroom. The pipe is suspended from the floor joists using the U-shape hanger thingies. Since the sound hasn’t happened in the past, I’m guessing one of those may be coming loose?

Before I crawl under the house to look for the source, I thought I’d check to see if others had other ideas of suggestions of what I should be looking for.

Thanks.

I'd live with it. I get a tick tick tick when I turn my hot water on. It's just a little bit of expansion.

I have tried to get rid of it by rerouting pipe but it always comes back.