Around 99 times out of 100, water hammer is caused by a worn washer shaft which goes into a hole  in your tap handle.

If it is a plastic shaft - then replace it with a plastic shaft with a fitted washer.

If it is a copper or brass shaft, place the shaft part on a hard surface and tap it with a hammer, till it flattens enough to fit snugly in the hole in your tap handle. If you flatten it too much and it won't go in, turn and tap it until it will, or file it down so it will fit snugly in.

That will virtually always fix the problem.

I met an older lady when i was a handyman who was quoted $3500 to fix water hammer. The plumber told her it was because of worn and loose pipes behind her walls and was a major repair job, plus she would have to hire a carpenter or a gyprock fixer to repair the wall when he was finished.

That was over 20 years ago and she was distraught at facing the expense, given she was a pensioner and didn't even have the capacity to borrow that much. I had done a tree lopping job for her and she rang me and asked me if I knew somebody who would do it cheaper.

When I came over and fixed it with the above method, in about two minutes flat, she was so excited and relieved and hugged me. Then she got angry with the plumber whom she rang immediately and abused him verbally. She tried to make me take $50 but i insisted on only charging her my minmimum fee of $20.

I got a few jobs from her recommending me to her friends and relatives as a result. Plumbers won't tell you that trick, but believe me, it works. if you ever get water hammer, try it!