1990 GMC C1500 whistling sound when engine is hot and conti

whistling sound when the engine is hot and continues
when in neutral. stops when I accelerate.
starts again when it goes back to neutral
engine is idling faster than normal when engine is hot

this is what I have done to correct this problem and nothing has worked.

changed serpentine belt

changed barings inside alternator

changed idler pulley

changed 90' on vacuum hose tip (end)

put starting fluid around hoses and under air filter to find out where the noise was coming from - couldn't find noise

thank you,
bbeerbower