Engine Performance problem
1997 Audi A4 4 cyl Four Wheel Drive Automatic 160,00 miles

I will be driving along, going 65 on the highway,or 25 in a neighborhood, and suddenly this loud grinding noise starts to come from the front of the car. My acceleration no longer works when this happens, so i have to put the car in Nuetral, turn the car off, turn it back on, and put it in drive. I have taken it to an audi dealer and 1,600$ later, and an "i dont know what it is" im still having the same problem. Yesterday, after it made the noise and I put it in nuetral and turned the car off, the noise still came even when the car was off. It is undriveable now, seeing as how I had to stop in the middle of an intersection to execute that horrid process. PLEASE HELP!