OK, so I was pulling away from a parking spot (truck already warm) and I hear the "tonk" sounds. I thought it was something with my suspension or steering. I drove slowly around the parking lot till I found an open space to crawl under and take a look. Nothing.
I drove on home with no further problems.
The next day I headed into town and about a hundred yards down the road I got a tonk or two followed by a squeal that sounded like rubber on steel.
I turned back to my driveway, took a look under the truck and again, nothing.
I headed on my way with no further problems that day.
Later, I checked under the hood, added a bit of tranny fluid, found a small vacuum leak in a hard plastic line that was rubbing on an AC line (fixed it with electric tape) and took a test drive. I got no noises this time with the truck warmed up again.
I'm just spit-balling here, but could it be that my tranny was not releasing from "Park"? Could the vacuum leak have something to do with it?
Any help appreciated.