My plumbing- A Very Strange thing happening

I recently changed the spindle and washer on the tap that controls the water to my unit from the mains. Since I changed it I started hearing an intermittent loud sharp knock coming from the cupboard where the pipe and tap are, which I have never heard before. It happens on average about every 5-10 minutes, a single sharp crack sound and then silence until it does it again. I was intrigued by this so I waited for the next sound with my fingers resting lightly on the pipe, and sure enough the sharp crack/knock sound is coming from the pipe and as it knocks the pipe vibrates quite fiercely for a split second. Then it is all silent until the next loud knock/crack comes. It is not the same sound as pipe hammer which can last a long time and only happens when a tap is turned on or off. It has never happened before I changed the spindle and the washer, so doing that must have something to do with it. It is not related to me turning on another tap in my unit, it just happens. I have run all the taps to make sure there is no air trapped in any pipes. It really has me mystified and it is damn annoying. What do you think could be causing this?


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