The buildup of scale or mineral deposits in the water heater may cause a rumbling noise. When this happens, allow a few quarts of water to run from the drain valve to remove the sediments.
If this doesn't solve your problem and you purchased your water heater from us, please contact us so we can help. If you purchased your water heater elsewhere, try contacting the plumbing contractor that installed the water heater.  You can also contact the manufacturer of the water heater.