rattling or backfiring?

[Festively Jolly]

hey list!  ok i gots me a very annoying problem.  my car is a 1988 audi
90 FWD and when i start it (outside temp does not matter)  it will
stumble and rumble and pit putt until about 5 min later when the engine
or exhaust warms up.  i cant tell what the problem is exactly.  it
sounds like the noise is in the exhaust, but what would that be?  its
only when i use about 1/2 throttle and no more than 5 min or so into the
drive.  anyone have any advice where to  start looking?  thanks ALOT
people.