The noise is caused by condensation and sediment buildup in the bottom of the tank. It's also an indication that it's getting near the end of its useful life. You may be able to prolong the inevitable replacement by flushing the tank completely.
First, shut of the gas or turn off the power (if electric) to the tank. Close the cold water fill valve going into the top of the unit. Attach a garden hose to the faucet at the bottom and run it into a nearby floor drain. Open the faucet, and drain the tank completely. NOTE: Be careful not to burn yourself, as the water will be HOT. Once the tank is completely drained, close the faucet on the bottom, open the valve on the top to refill the tank. Once it's full, turn the gas/power back on.
This process should be done once per year on all water heaters.
Hal, April 2008
