I assume it is a Mk 1 punto?

If it is then the radius arms would be the most obvious cause, but as
you have replaced these, then it is unlikely.  Nevertheless, have
another look.  Two good ways to check are 1) drive behind the car when
it is on the road - both rear wheels should sit at 90 degrees to the
road surface.  If they lean in, then recheck the radius arm again. 2)
put a wheel nut lever on one wheel nut and lean on it as if to take the
wheel off - do not loosen the nut.  Any play will show up as wheel
movement as you lean on the lever.

Next, check the antiroll bar and its supports.  Use a lever to stress
the bushes.

Check the four subframe mountings.  Fiat do a bush reinforcement as a
modification that fits over the original bush - takes 5 minutes to
install.  As far as I know, the original bushes are an integral part of
the subframe and cannot be replaced - well done FIAT some rediculous
design!

Next, check the brakes.  It is possible that the shoes have worked
loose, but this would not make a long banging.  Check anyway.

Have you checked the spare wheel and tool holder?  Have you checked the
springs for broken coils and are the shock absorbers still working. Do
the bounce test - you should feel immediate resistance to forced
bouncing.

Apart from that, there is not much else at the back of a punto.