The jetta 2.8L VR6 uses a metal timing chain, intended to last the life of the car. But the chain rides, and is tensioned by several plastic guide rails. It is common for the guide rails to fail before 100,000 miles. When they fail, the timing chain will at best make a lot of noise - like marbles in a can. But the risk of the chain skipping a tooth or more on the sprockets is high and will throw off the timing. This will cause the car to run rough (if at all) and acceleration will be VERY poor. Because of the "interference" nature of these engines, a mis-timed engine can cause valve and/or piston damage which is an expensive repair.
My Jetta is going through repair of the timing chain guide rails now. It requires removal of the transmission - no small thing.
the engine made a loud sound as I was driving 20 MPH, a few blocks later the car stalled. It would not turn over and had a whining noise when I tried to restart the engine... It was towed to a VW dealer who said they would have to pull the head off to fully diagnosis the problem. They saw the timing mark was out of align and estimated $1500-$4500. If there was valve, head or piston damage those parts would be extra plus the timing chain parts.
Its was a good car, very clean and reliable. I hoped to keep it a few more years....now it just don't seem to make sense to put that much money into that open ended repair estimate. The repair shop said they never saw a car with that low a mileage ever have that type of problem.