[spongebob]

Depends what loco you are referring to i suppose. My experience of the old mainline warships was they all did it up to the point where the drive gears on the axles wore a bit and the motor just sat there spinning. Could be a tooth or 2 missing from one of the gears or a worn mechanism. Give us a bit more info and someone may be able to help.

 

[spongebob]

It could be the same as the old mainline OO gauge warship which i have had 3 of and all did the same noise i've also seen them for sale on ebay making this noise. I think it may be an attempt to replicate a diesel loco sound as to my mind it does sound like a clanky old leyland national bus engine ticking over. Like you i haven't been able to pinpoint it to an axle or anything.

 

[spongebob]

Tyre replacement isn'y usually a difficult job just make sure you do it in a confined space in case a tyre decides to launch itself across the room. Very annoying and always difficult to find.