If it only happens when using small/small combinations, then its likely the chain hitting the pin that connects both side rails of the front der. Check to see that the chain legnth is correct, too long a chain will give too much slack which is only evident when you backpedal.
But be careful if you need to shoten the chain, you have to set the chain legnth to make sure you have enough chain for the big/big combo, if its too short for that combo, you will rip off the rear der.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1