Was sitting in a drive thru the other day and threw my truck into park and it started making this strange noise. Its almost like a buzzing/grinding sound. I cant quite narrow it down, it seems like its coming from the transmission but it echos off everything underneath the truck so that's my best guess. It only does it in park and neutral and its random in pattern. The truck is a 2005 3.5 z71 at 235,000 miles. I'm getting the trans fluid changed in a couple days as its about time for it. Anyone have any ideas what could be making this sound?