My 1999 Audi A6 Quattro, 2.8L AHA, Automatic.

Rear end is an open diff, I took it out because that's what I thought the noise was coming from but didn't see any signs of wear. Really confused now because I was certain that was where the problem was.

Ticking noise is very muffled and comes from what sounds like the back end. Can't really tell either left/right so I assumed center. It sounds like a gear is slipping under higher power and up hills. If I drive nicely even at higher speeds it won't make any noise. It only makes the noise when accelerating quickly from a stop or passing on the freeway. Speed doesn't seem to change the rate of clicking, only more commanded power makes it faster and less power makes it slower. The sound really isn't too loud, and actually I can almost not hear it at all if the stereo is on. You also can't really feel anything in the car, no vibrations or hesitation.

I talked to the dealer and they said rear differential problems with Audi's is VERY rare... like they have almost never had a case. They suggested maybe a slipping axle shaft, but I'm not seeing any wear on the splines to suggest that. CV joints seem to be in great condition, the car only has 112k miles on it. I really don't feel like spending $122/hr for the dealer to find the problem. Especially being this diagnostics requires 2 mechanics meaning double time.

Anyone ever had this issue or something similar? Maybe an issue with the center differential? I've never taken one apart so I really don't know much about how they are built.