As Eric has said, this is almost certainly being caused by sediment build-up. The sediment is wet, and when the element turns on, it turns the nearby water trapped by the sediment into steam, and it pops. It's not very much different than popcorn popping.
I've copied the appropriate steps from another answer of mine (didn't want to link it since you only need to do about half of what the other guy needed to do.)
Flushing your water heater:
- Unplug your water heater.
- Attach a garden hose to the drain at the bottom of the water heater.
- Run the other end of the garden hose outside.
- Open the drain.
This will forcefully drain the water heater, and hopefully the city's water pressure will be able to get a decent amount of the sediment out of the bottom. Depending on how fast the water is flowing, let this continue for 1-5 minutes. Sometimes the drain can be pretty clogged up or the bottom quite full, and the water won't come out very fast, especially when you disconnect the city's water pressure.
If this does not solve your problem, you'll need to be ready to replace the water heater (the bottom element will fail if buried under sediment, and no bottom element means basically no hot water)... or you can do a more thorough cleaning by taking the bottom element out (see other answer for instructions) and wetvac'ing the sediment out, like so: www.youtube.com/watch?v=pt1M7qO6Lh8