ummm nope no clue... it stops though at some point.. cause by the time i go to the car in the morning it doesn't make the sound.... Best thoughts so far is fuel pump, or pressure release from something like fuel tank or something.. maybe heat from exaust .... but its almost perfectly timed.... so... still not sure.. but it stops at some point..

 

yeh it stops like it dosnt do it everyday just here and then.
no harm to me but just seems to ge louder. its like a phone is ringing on vibrate or something.

 

listening under the car it sounds like its coming from the rear center under the car.