Loud banging noise in drainage pipe...

I live in a 3 story condo building and have had very loud banging in the pipes from both my own home as well as my upstairs neighbors. The noise occurs mostly while using the hot water in the shower or bathroom sink. The noise stops shortly after cutting off the water. I had a plumber come out last week and he tells me that the noise is coming from the drainage pipe, something about it expanding when the hotwater runs through. Does that sound likely? Any suggestions? He says the plumbing cost will be about $250 (in the Northern Virginia area) but probably another $250 to repair the drywall.
Thanks for any help!
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I can't say I agree with your plumber. A good way to find out would be to get a bucket of hot water and pour it into the drain. If it's the drain pipe, the hot water from a bucket vs. hot water directly from the tap is still hot water..and would still cause the same noises. You can test it yourself. But it's not your drain lines "expanding". Frankly, cast iron drain lines aren't going to "expand" when you dump 125-degree water into the drains. And just out of curiosity, did he explain to you why the pipes were only expanding in your bathroom? I would think that they would expand when you used the kitchen sink or any other hot water in the apartment. Expanding cast iron..yeah... stupid explanation = "I don't know" = call another plumber
You haven't described the sound. You might need a hammer arrester.
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