Im not sure what could have cause this but for about a month now my cars has been making this humming sounds and the car kind of vibrates when it idles in park around 900rpm even though it should be idling a little lower, and when im at a stop in "D" the idles is at ether 1000rpm - 750 ish rpm (where it was before this started happening. im not sure if this has anything to do with it but the thermostat gasket is leakin too, and that was just replaced...

 

does your steering wheel vibrate too?

possibly motor mounts

 

mine sc did this for a while i got it up on a lift and the exhaust was just a little loose from a hanger coming part undone. thats just my experience tho poke around ... let us know