Whinning sound fro start to 40 mph

2000 jimmy whining sound coming from under dash at start to 40 mph-after that you hear engine. As you slow down 40-10 mph you hear it again(sounds like air liner engines winding up. Rear pinion seal and outer bearing was done left from hub assbly was replaces all wheels seem to have the right amount of play in them 4wd works fine what can it possibly be! Driving me up the wall! Some one told me it was the alternator. Only makes the sound when everything starts turning forward- revers it's not heard at all