The most common cause of cracking noises associated with hot-water is the thermal expansion and contraction of metal pipes clamped against woodwork.
As the water in the tank cools, your heater will periodically come on and re-heat the water, even if you have not used any hot water. This thermal cycling can cause metal parts to heat up and expand. This can cause a range of sudden sounds from quiet clicking to louder cracking.
The only solutions I know of are to review the type, positioning and number of clamps used to hold the heater and any pipes from the heater to the taps/faucets.
I guess insulating the tank and pipes might help a little. I'd maybe try very-slightly loosening any pipe clamps and checking if the pipes run directly against woodwork anywhere.
It is also possible the cause of the sound is internal to the heater, in that case you can only wait and see if it gets better over time.