Re: Weird intermittent humming noise...
Could be brake related... remember when in Drive (vs. 'L') the brakes are applied when you let off the gas, a.k.a "Brake Assist", so it could be a slightly warped front rotor. I had to replace mine @ 28k miles, but than again, you can typically feel warped rotors through the steering wheel under braking.
During this event, are you getting any feedback through the steering wheel?
'09 XR, AWD, all options except rear DVD entertainment