Fan is making a rattling noise

This can be caused by a worn sleeve bearing; these are commonly used and while quieter when new, wear out faster than a ball bearing.
It may also be caused by an imbalance of the fan blades caused by dust buildup.
With the unit off and a panel removed so you can access the fan blade, gently try rocking the bladed fore and aft to see if you can feel excessive wear.
This type of motor isn't expensive to replace and likely to be a 'commodity' type that can be bought from various sources. If in the US, look for Johnstone Supply in the yellow pages under Appliance Parts or on line. If you have a photo of the motor or bring it into a parts house, they will likely be able to hand you the replacement.
Most of this type of motor is mounted with as few as two screws/nuts and easy to replace.