Once I turn on the water it runs fine for a few seconds then the flow slows to about half and the banging begins. If I open the cold water valve on the bathroom sink directly behind the problem sink and then open the problem valve there is no banging. I have drained the system a couple times and the one valve still has this problem. Would changing the valve stem and seat resolve this issue. The problem began a few months ago. All the plumbing was redone about 6 years ago. I don't understand why just this one valve would be acting up. Thanks for you help

More details please! What fixture is the valve on?
Is it a 1, 2 or 3 handle"
What 's the brand name?
How old is it?
Answer back with all the details you can furnish me,
Cheers, Tom

The valve is on a bathroom sink. Two handle . The brand name is Sigma. It is approx. 10 years old. It was installed about 6 years ago. When I finally got to the renovation. I recently swapped out the hot water stem and seat because it was very stiff. Big money fixtures, big mistake.
Thanks Tom

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Once I turn on the water it runs fine for a few seconds then the flow slows to about half and the banging begins.

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Sounds like seat chatter" to me. Seat chatter occurs when either a small piece of trash or a washer has split and is vibrating against the seat,'
Try this. Shut the water off under the cabinet and remove the handle. You will see a large nut with the stem in the center. This will be the "bonnet packing nut". Unscrew it and back the stem and washer assembly out counter clockwise. Check the washer and replace if necessary, Now, ( and this is important ) place a pan over the open valve and turn the water back on to flush out the supply and valve body. Reassemble and test, Better now? Good luck, Tom

I will check this out and let you know Thanks Tom