DistanceGuy1

At times when I just get in the car (haven't started it yet) I hear a loud humming sound (10 seconds) from the rear of the car. At other times I hear this same humming sound as soon as I put the key in the ignition. Any ideas what this sound is?

zimaad

one possibility is that the compressor for the adjustable rear air shocks is running. I think normally, it runs for a few seconds after the car is started for about 10 seconds. This would also happen if you turn the ignition half-way and let it sit there, but not actually attempt to crank the car. I wouldn't think there would be any noises without a key in the ignition. I could be wrong and the auto level rear suspension can kick on when you sit in the car, but I haven't heard of that.

OEM Only

zimaad is correct...the sound you hear is the air compressor adjusting the rear shock level. When you turn the switch to the on position, the ELC (electronic level control) system purges residual air from the system. When you start the engine, after 45 seconds, the ELC will determine, through the positioning arm in relation to the ELC switch, if the load in the car is unbalanced and it will allow compressed air to enter either shock or both to compensate for the load and level the load. That is the sound you are hearing emmanating from the rear of the car.