whistling noise
Hi there,
As big daddy said, it could be a vacuum leak. I think if not a vacuum leak, it could be the cyclonic valve (basically an oil separator, or like a PCV valve) located under the intake manifold, beside the throttle valve.
It's not an easy job to replace this piece.
If there's a vacuum leak, you can use brake cleaner or even WD-40 sprayed liberally over the air intake ducts, and listen for a change in idle.
Good luck,
Tiger