Tranny grinding?

[katz]

The '96 Tahoe is having an issue. The Odometer just rolled to 100k this week, and of course all hell breaks loose.

 

Here are the symptoms...

1) When applying more than "very little" throttle there is a grinding/rattling noise coming from the transmission. Even when gas isn't applied and the truck is coasting, it grinds, but it doesn't always grind.

2) When experimenting in the driveway (revving it up while in neutral, to see if there was something with the driveline like a U-joint instead of transmission) I found that it does NOT grind when I rev it up in neutral. BUT when I was stopped on a level surface and I was shifting into reverse to line up to go up the rhino ramps it started to grind when I wasn't moving at all. While it was grinding, the speedo moved up to 20mph, even though I hadn't applied any throttle, my foot was on the brake, and I was standing still; the only change in the car is I put it into reverse.

 

The car doesn't have a speedo cable, but I'm wondering if the idler gear, or gear which feeds the electronic pulse of the speedo became chipped or something. I don't think it is anything big like the torque converter. It still manages to shift and change gears without problem. The gearing for the speedo seems to be in the tailhousing of the tranny, which means the whole tailhousing would need to come off/apart to fix this. I see a high maintenance bill in my future. Ugh.

 

Any idea as to what this is? Should I let a stealer service this or can I trust a local mechanic?