just recently got 2002 bone stock mustang v6 automatic, and this just happened to me the other night. as i got home and put my car in park it made a loud grinding noise, so i quickly turned it off and then it kept making the noise. so i turned my lights off and it quit. took my keys out then turned the lights on and it started to try and crank up with the lights on. turned the lights off and it stopped. now in drive and reverse it doesnt make the sound, only in park and neutral. it is not my trans. any idea

 

Get a recording so we can hear what it sounds like.

 

sounds like you might have a wiring nightmare that is engaging your starter when it shouldn't be engaged. not sure where to point you as far as fixing it.