Class 90 "popping" sound

[John745]

There most likely will have already been a thread for this, but what exactly are those popping sounds on a 90? (in a bit of detail if possible)

If there's already a thread answering this then please link it - thank you.

 

[Flying Snail]

Spirax valve, removes moisture from compressed air system.

 

[John745]

Thanks a ton - I've been wondering for quite a while!