How do you stop a noisy hot water heater?

Answer 1

You don't. If it is a gas water heater and you have hard water, the sound you hear is the lime deposits being heated and releasing gases. The only way to fix this is to buy a new water heater and install a water softener. This is also true for electric water heaters.

Answer 2

You probably need to contact a plumber about this. There may be some gas trapped in the pipes, or there may be other problems. If it's benign, the best thing you can do is attempt to sound-proof the boiler cupboard. You can use any soft material for this, but it needs to be in thick layers. Egg-boxes pinned to the inside of the cupboard wall should also help absorb some of the sound.