Why the first successful steam engine was called puffing billy?

The first commercially successful steam engine was Newcomen's atmospheric engine, a type of reciprocating piston steam engine that used atmospheric pressure to deliver the power. This engine, developed around 1712, did not have a puffing sound and the name "Puffing Billy" or "Puff Billy" is usually associated with a later engine. One of the earliest famous working railway locomotives that carried the name Puffing Billy was Richard Trevithick's locomotive built in 1813-1814. This early steam locomotive ran on the Wylam Colliery Railway in Northumberland, North-East England.