chuff sound on steam locomotives

[canadiannational1996]

has this happened to anyone one,when a steam locomotive starts moving it chuffs two times and then doesent then again but when you go inside the cab there is regular chuffing.what may be the problem,if anyone can help thanks

[thecanadianrail]

only with the default RS locos and locos whom use those sounds. (black 5 steam loco)