The heater in my 2001 Aurora whistles more and more loudly. Turn off the heater - whistle goes away. It is still working, but has become annoying. How do I get at it? Also, is this vacuum or heater core related?

Odd times there is leaf build up by the heater core
that could caused the whistle

I don't think its related to vacuum or heater core (unless the heater core is leaking coolant - this would be noticeable as steam & coolant loss to the vehicle interior).
First, try to ascertain if the whistle is directly related to fan motor rotation - if so:-
A likely cause is dry bearings on the heater blower motor, but a similar whistle can be caused by debris such as a leaves/twigs fouling the fan airflow or drum.

If the bearings are dry, it is generally not easy to access the bearings to replace or lubricate.

Sometimes a small hole can be carefully drilled adjacent to the motor bearing mounts, allowing a lubricant such as WD40/penetrene to be applied to the bearing area.
Good luck.