Welcome to S1 ownership!
You shouldn't notice any more noise going backwards than forwards !
Is it simply the transmission clunk common to S1's or ???.
You'll get used to the transmission snatch after a few miles and learn to avoid it.
Keep us posted.
Enjoy the car, they are sublime!

1969 4/4
1995 plus 8
1996 Plus 4 4seater
2002 Aero S1
2013 M3W
2014 Plus 8