My hot water heater (gas & 9 years old) has been making popping/knocking sounds periodically for a while now, and just lately the water has been getting too hot. Do I need to worry?

This can be a sign there is a buildup of sediment in the bottom of the tank.

The 'popping' sound is the bubbles released as the mud on the bottom boils.

The water heater will overheat trying to compensate for the fact the mud insulates it from the interior thermostat.

Have the tank flushed and your troubles should cease.

Manhattan24 nailed it!
If you do not flush,(not drain) your heater on a regular basis it will build up mineral on the bottom of the tank. When the flame comes on it turns the water trapped under the mineral to steam and the steam escaping is what you hear. Let me show you how to flush your heater. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) and some white grains,(lime or calcium carbonate). The whole thing shouldn't take more then a few minutes. Don't forget to flush it out every month. Your heater will thank you for it. You might want to consider cleaning the screens on your aerators if you have a high mineral buildup. If the problem still persists after you have flushed, you will have to drain the heater, remove the boiler drain and go up in the bottom of the tank with a wood or plastic rod, (not metal) and try to break up the mineral and flush it out. Good luck and thank you for rating my answer, Tom

Agreed! Do you have a water softener? That would help decrease the nasties in your water.