Thunder is the sound caused by lightning. Depending on the distance from and nature of the lightning, it can range from a sharp, loud crack to a long, low rumble (brontide).
The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning.
The air in a balloon is at a higher pressure than its surroundings because the elastic tension of the balloon skin is pulling inwards. ... The high-pressure air that was inside the balloon is now free to expand and this creates a pressure wave that our ears hear as a bang.