1992 cavalier- engine revs up and down

make sure that you do not have any vacumn leaks , and also unplug the vacumn hose that goes to the egr valve and see if the problem goes away ,, as far as the humming noise , sounds like you might have some wheel bearings going out in it ..

the car idle is what hums up and down, not the wheel bearings, will check the vacuum, but was thinking maybe a idle sensor or timing belt, or relay of some sort