The only time I have heard railway engines 'ticking over' when stationary, is in films or documentaries.
I didn't hear it at the Severn Valley Railway. I just took the opportunity whilst there to ask a question about it to some of the railway enthusiasts present.
I am sorry to have misled everyone.
But the following from chuffchuff and locojoe is very close to the sound I remember.
http://www.youtube.com/watch?v=VRhORndOWzk
Many thanks
Keith