High pitch humm from engine

Like a sort of whistle sound? High pitch like that is a vacuum leak, and when you accelerate it will not be heard if it is a vacuum leak. And that is a boat~load of possible sources. Check your power steering fluid. that makes a distinctive growl sound, not high pitch but no harm in checking

Possibly loose or aged belt or idler pulley. Can use a section of hard vacuum cleaner hose or PVC pipe to help locate where sound is coming from. With vehicle idling hold one end of pipe to your ear and carefully move other end in and around engine compartment. The sound will get louder the closer you get to the problem. Use caution not to get pipe in tangled in any of the belts, pulleys or fans.