Chances are this is related to sediment build up. A flush of the water heater may be all that is required.

Follow these steps to flush your water heater:
CAUTION: The water in the water heater is very hot. Use extreme caution and keep children and pets away from the area when flushing the water heater.

1. Attach a garden hose to the drain valve (looks like a hose bibb) at the bottom of the heater. Run the garden hose either outside or to the nearest drain.
2. Open the drain valve, which will allow the water to drain through the hose.
3. When the water is running clear (usually after about 20 minutes), close the drain valve and remove the hose.
4. Turn on a hot water faucet in your home and let it run until all air bubbles are out of the line, then turn it off.