@feha
You generally won't hear the chimney at all since it's just uncompressed hot air escaping. The sound would be from compressed steam exhaustion in puffs, which carries a fizzle or a hiss with it.
@tanterei yep, that is why I am saying it is probably closer to po than choo. Well, that and the fact that choo and other high-frequency sounds is hard to make without much smaller cross-section, so even if a build-up occurs (puffs of smoke rather than continuous smoke) it would not become a choo.
Also, I highly doubt there is compressed steam (or uncompressed for that matter) in the chimney like you claim. Most likely that smokestack is dedicated to the smoke produced by burning coal, since it is black.
Engineering stuff will be in awe by the guys wherever they are.
Even a Dam will make us stay there for hours staring the water flows and thinking how it works...
Its the guys romance...