The LNER Encyclopedia

On a trip to the ELR this weekend I was fortunate to ride behind this wonderful steam engine As I was gazing on in wonder as she backed on to her train I noticed a very high pitched whining sound rising in intensity coming from the motion or that area ! It didn't seem to produce any steam and sounded more like a very powerful vacuum cleaner starting up and switching down as she got underway ...can any of you out there shed any light on this ?
I am guessing it might be braking related ????