See Video Below
To simply drain a water heater does not remove the scale and sediment resting below the drain valve.
When this debris thickens enough the hot water will begin to snap, crackle and pop in an effort to escape the bottom.
To remove this stuff the water and gas lines or electrical lines must be disconnected so the rinse water can be shaken and the tank can be leaned over.
This water heater would not drain so I moved it outside where I could replace the drain valve with a full-port ball valve and poke around.
I then put it on a cement block/stair step so I could rinse it out more easily.