When I turn on the shower, after a minute or two you hear a "clunk" in the pipes and the hot water spurts out suddenly. What is the cause of this problem?

Need more info... what type pipe? What water pressure? What type faucet? Only on this faucet?

The pipes are copper. The townhouse was built in 1979. The water pressure seems to be normal. The shower fixtures is Delta brand. The hot water spurts just happened a week ago. When you turn on the shower, it's fine and then a minute or two later, you hear a "clunk" and then hot water spurts out of the shower head so you have to immediately move aside and turn up the cold water. Thanks

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