If the sounds are coming from the motors, then no matter which
line the train is travelling on, there will be such sounds.
For the video on the second post it would most likely be caused
by worn bearings in the motor or along the connecting rods.
Faulty motor sound wise, many things can go wrong in the motor.
Shall not elaborate too much, but basically it has to do with
windings and bearings inside the motor. After some time of
wear-and-tear, strange sounds will start to emit and its time to
send the motors for a health check.
Other than the motor, voltage controllers/regulators (GTO and
IGBT controllers, most commonly used) may also emit irregular
sounds if they are of a unhealthy condition, but normally these
non-moving parts, when emitting noise, will deem a bigger problem
than motors.
Last but not least you have flat-wheels...