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Hello and thank you for your post. I am a master certified GM tech, with 15 years Saturn experience,and can help you to resolve your issue.
if you put the vehicle up in the air (all 4 tires off the ground) and drive it at the speed that makes the noise you can use either a long screwdriver pressed to your ear or a mechanics stethoscope and listen to the rear differential, rear hub bearings and driveshaft center support bearing
(if it is awd)
if this is 2wd than you must spin the rear tires by hand while listening
one side will be significantly noiser than the other
unless this is a noisy tire this should lead you the noise
when listening to teh rear bearings, place the listening device on the bolts that secure the bearing to teh hub
you need to come in from behind
they are 13mm head bolts
4 of them
any one will do
that is where a noisy bearing will sound the loudest
hope this helps
(bearings are very common to go bad on these)