It sort of sounds to me like someone sawing real hard and fast with a metal hacksaw.
Whatever it was, it didn't seem to bother the locomotive crew enough to cause them to stop the trip , or maybe they just didn't have any options but to go.
We had a weird one this weekend, we were running our locomotive (former Monson RR #3) and it was making a faint whistle which was perfectly timed with the valve events.. at first we thought it was a squeak, but later realized it was the sound of steam blowing through a very small aperature (literally a whistling ). Later in the day, we pulled the throttle real hard and a rag used in winterizing the engine a few years ago came flying out ! Someone had lost it into the saddle while trying to get the water out.
Regards,
Trevor H.