My furnace is 8 yrs old, forced air. Turned on air conditioner today, it is whistling? Never did before.

Look for a door lose or a tape seem lose. You have some air coming from somewhere it was not before. If you can not find any thing it may be the blower motor its self.

Also change your filter... it could be clogged or dirty or not sitting in there correctly and causing airflow to change.