I’ve noticed water pipes start to bang over the last couple of weeks. This occurs when the showers are running in different bathrooms and not when faucets in the sinks are used. What would be the reason for this? Is it a seasonal thing?
Can you turn down your water pressure at the valve? Water hammer can be caused by high water pressure or lose straps, etc. There are a few solutions outlined in this article
https://www.allclearplumbingupstate.com/will-an-expansion-tank-stop-water-hammer/
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Thanks @Marquis. I’ll try those suggestions.
I’ve read about water hammer. I guess I don’t understand why it happens suddenly. Does something change in the copper composition due to seasons?
When air bubbles go in the pipes, water hammer happens. You can try emptying the pipes and slowly refill with water. Easiest is to put in pressure regulator, but that would mean lower water pressure. Valve pressure regulator rarely works but you could try that too. Find a trustworthy plumber and he will explain & do minimal work for you.
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We experienced something similar a few years back when we had repiped to copper and had significantly improved water pressure. I went into the crawl space and rechecked to make sure all pipes were securely strapped. Didn’t find any unstrapped pipes. Was able to narrow down the problem to the shower valve cartridge. We changed out the shower valve cartridge and the banging/thumping went away.
Another common fix would be to install a water hammer arrestor.
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So dirty 
Not a task that white collared SWEs would want to do.
Some more fix-it tips here (if there is a clogged pipe):
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You may be dealing with a water hammer or hydraulic shock. It could be because of high water pressure or there are some loose or faulty valves. You can try to check your water system for any of the two.
If that doesn’t work, then it’s time to all a professional plumber.