Kieran,
The sound you describe that decreases as the motor warms up could also be caused by the motor bearings going bad. As time goes by the grease inside the bearings dries up causing friction inside the bearing that makes noise and slowly destroys it. The noise is loudest when the motor is cold.
Replacing the bearings is tricky and requires special tools. Unless you are experienced in working on electric motors, it would be best to bring your 514 to a Kirby service center or a regular vacuum shop that services Kirby's.
It may also be motor brushes as Ben states above. These are easily replaced by you.
~Stan