Could be belt, but try checking to see if the bolts on valve cover are tight, and also bolts on intake manifold, -- if they are sucking (vacuum) in higher at low rpms, and will make a whistling sound as gasket vibrates on its seat! I had a Plymouth Acclaim that did that and because the hose to pcv valve was kinked - it was sucking shut at high vacuum!
Squirting water or WD-40- on contact surfaces of belts will stop squeaking for a few minutes, - confirming it is belt tension (or glazed belt)!