Hi all!

This has been driving me nuts for a couple of hours so I figured I would ask the experts. Could anyone tell me  what is making the noise when the train is at idle in this scene from Butch Cassidy and the Sundance Kid?

it makes a distinctive rhythmic sound around once a second. Some research tells me that this could be the air pump/compressor. However I haven't been able to find another steam locomotive that sounds like this at idle. Any help would be much appreciated, thank you!