I've been racking up the miles on FGW HSTs over the last few days (especially today where I've already clocked up six hours and there is another two to go) and I've noticed that whenever they're under breaking or standing in stations the bogies emit a very loud whistling noise that sounds very similar to a boiling kettle. I've never noticed anything like this on EC or XC HSTs so I'm wondering if anyone can explain the reason for the noise (and why it doesn't feature on other HSTs)?