Audi 100 Starting Problems

[Geoff Doctor]

Well you guys helped me get my audi started before, so I figured I 
would ask again. Well it finally quit firing (guess I got a bad 
Distributer Cap from the junk yard) So I went out and bought a new cap 
and rotor from Pep Boys. Installed it, and the car fires and cranks, 
but before it will start something makes a loud bang. Also if I try 
cranking it when the battery is low, after the bang it will stop trying 
to crank at all. Could this be a started problem? And if so, where is 
the starter on these vehicles. I heard rumors that you have to pull the 
tranny on some Audi's to replace the starter, but I don't have the car 
here right now and cannot look for myself. Any incite to what the bang 
could be, or why I can get it started is appreciated.

Thanks!