Reving in Park, Squeaking when Slow?

Check the tension on your engine belts, sounds like they are slipping its quite common, squeals when under load and when the load is removed it'll stop.. most probably alternator belt. Check that the belt is tight and not worn out and its sitting in the pulley correctly. Also the noise the engine is making without hearing it i'd say that like most cars the engine idle picks up when its unloaded (not in gear), so when its in gear it'll have a different noise like the engine is labouring alittle, and when that load is removed it'll rev higher.. Also just check on the throttle body that the spring return on the throttle cable is not being restricted.