What causes the hammering noise when you turn on the hot water pipe? Even when you turn it on slowly, it still hammers out a loud noise. It is happening at my Dad's house and I'm sick of him not trying to do anything about it. I know it's been a bit cold laterly. Come to think of it, I can't remember if the hammer noise occurs during summer. I'm afraid that I do not have enough money to fix this problem. No wonder my Dad is doing nothing.