Hammering pipes

In order to clear a sink drain clog in an upstairs bathroom this weekend, I needed to remove the shutoff valves to get clearance to remove the P trap. This entailed turning off valves in the basement for the lines that supplied the upstairs bathroom. Everything went back together again and, after some initial spluttering to rid the air in the lines when I first turned the water back on again, all seemed well. However, sometimes if I open the faucets only part way, a hammering starts in the pipes and the water turns off/on about 4 times per second. If I open the faucets further, the hammering stops. This happens in the sink faucets and the shower, which are fed by the same supply lines and happens in both the hot and cold sides. This never happened before I shut off the water.
Is there anything that I can do to get rid of this problem without having to install water hammer arrestors? Since this never happened before, I don't want to tear into my walls if I don't have to.
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MPost wrote:

In the flow of the water, the washer is free to move and slams into the seat and then pops back off, shutting off the flow several times a second.
You won't have to tear into any walls, but repairing the valve can be tough. You'll have to shut off the Main valve and drain the piping, then wrench the bonnet off the valve. The bonnet nut can be on really tight.
If the lines are copper, have a backup plan to install a new valve in case things go badly because the house will be without water.
Jim
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