On the 1952 Weybridge service having to put up with terrible screetching sounds from the train's suspension on a Class 450. SWR cutting maintenance again ?
If it was on straight track and the ride was uneven - tell the train crew.
If it was on a curve, probably the track mounted flange lubricators had run dry.
If it was raining and in a cutting, wheel slip on mushed leaves.
Anything else, someone listening to an on line streaming of a female "celebrity". ( I knew no good would come of on train wi-fi!.)
PS: Or the mice who probably feed quite well on all the dropped food .....
An increase in gradient can also produce a slight squeaking/rubbing noise from the bogies