The sound that is made when something heavy falls or hits something else. (Cambridge Dictionary)
A dull sound, as of a heavy blow or fall. (Dictionary.com)
A blow causing such a sound. (Dictionary.com)
A thud is a dull sound, such as that which a heavy object makes when it hits something soft. (Collins Dictionary)
If something thuds somewhere, it makes a dull sound, usually when it falls onto or hits something else. (Collins Dictionary)
A dull, heavy sound, such as that made by an object falling to the ground. (Oxford Dictionaries)
Move, fall, or strike something with a dull, heavy sound. (Oxford Dictionaries)
Comic book style Thud – Illustration.
Back to Earth Comes Space Balloon, With a Thud.