Specific speed, specific gear, specific road surface wet or dry, temperature related, while accelerating or decelerating, worse with windows up or down, A/C on or off, start up/warming up?? Could be a wheel bearing at specific speeds, engine related if gear dependent, a bad engine accessory like a water pump at a specific rpm range, a vacuum pump, or even a hum in the radio.
Wasting the dealer's time unless either he can duplicate or owner can provide with specifics relative to creating the hum. If it ain't humming when they test it as per the owner provided info - it's not humming.
Wasting the owner's time if they try to pursue a lemon law complaint. "Random" and "hum" with no specifics, will get you nothing and a waste of time in a lemon law claim. And after 8 visits nothing in dealer satisfaction either unless they are able to reproduce it or at least specifics relative to its"random" appearance. If the dealer was worried in the slightest about a lemon law complaint or lawsuit he would have had GM field rep present before the last repetitive visit for the same repair required by the State (brochures routinely included with new car owners manual package - most are 3+ strikes for failure to repair the same complaint) before owner being able to pursue a valid lemon law complaint for reimbursement by GM.