Hi Jim
Without being able to hear it - it is difficult to diagnose :)
The whistle could be due to the motor bearings being slightly dry.
This is the most probable cause.
I would be inclined to back up anything that you can't afford to lose - just in case.
A dying hard-drive usually "clicks" - this is a totally different diagnosis.
If you can put up with the whistle I would be inclined to leave it for a while.
It could, over time, sort itself out.
If it gets worse - then time to replace it.