My wife’s 06 Malibu just started this humming sound and it sounds like its coming from the drivers side when driving. When i look under the hood with the car running sounds like its coming from the alternator. When parked and rmps are increased.
The bearings in the alternator could be going bad. First, I’d check the belt and tensioner and any idlers to make sure the noise isn’t coming from there. You could remove the belt and start the engine to see if the sound stops. Don’t drive around like that, of course. If the sound stops, you’ll at least know the problem is something on the belt (alternator, compressor, tensioner, idler, etc).