The reason why you hear a delayed "rumble" after the flash of lightning is due to the difference in speed between light and sound.
For example, if you hear a rumbling sound 10 seconds after the lightning flash, you are 3400 meters away. If the sound is heard less than 3 seconds later, it can be calculated that the lightning has struck within about 1 km from the location. Sound can usually be heard up to about 10 km away. There are also cases where no sound can be heard even though the light is shining, and the distance in this case is about 40-50 km.
The rumbling of thunder is caused by the sudden heating and expansion of the air in the path of lightning. Air is essentially an insulator that does not conduct electricity. However, the energy of a huge thunderbolt tears the air, which is an insulator, apart and tries to reach the ground.
Lightning heats the surrounding air to about 30,000 degrees Celsius in an instant, increasing the pressure and causing it to expand at once. 30,000 degrees Celsius is about five times the temperature of the surface of the sun. The shock of the explosion is transmitted to the surrounding air, causing it to vibrate and produce a tremendous sound. When lightning strikes near you, you hear a "bang! or "crunch! or "crunch! The sound of thunder in the distance can be heard as "rumbling" as it reverberates in various places such as clouds and mountains.