Strange noise and bouncy speedometer needle

[71threequarterton]

So I got on the road to work this morning and all of a sudden I started hearing a loud noise which sounded like radio static at first. I reached over and turned off the radio but the noise didn't go away. With the radio off, I could hear it a little more clearly. It sounded like higher pitched grinding, rotating sound, much like the pitch of radio static though. As I came to a traffic signal the rotational sound slowed down and stopped as the truck came to a stop. At the time this was happening, the speedometer needle was going nuts: spinning all the way well past its limit and then back down to zero and just bouncing all over the place. As I accelerated, the noise came back and slowed down as the truck slowed down and then stopped when I came to a stop. The whole time, the speedometer meter was bouncing around erratically. Does this sound like some kind of gear drive that has mostly disengaged or has ground down to the point that it barely engages? I've never heard anything like this before. The motor seems to run fine regardless but I drove it home and parked it because I have no idea what this is or what I might be making worse if I drive it. Had to drive the wife's car to work