Is it not just the door pin that makes the noise as well?
As you open the door.. there is a Shock caused the detent in the arm passing over the sprung roller.
this should sound like a quiet click..
if the pin is loose and bearing in mind that the spring is very strong..
the Shock it amplified by the movement between the arm and pin..
in fact the door acts like a sound box .. kinda like a guitar..
anyway the result is a loud CRACK!
eventually this will cause the check strap mechanism to fail altogether..
So if your check strap works, but it is just loud.. look at the pin
If your check strap is totally knackered and does not keep the door open.. replace the check strap mechanism..
Phase 1 cars have a removable/replaceable pin (roll pin).. which I think is great
phase 2 cars have a fixed pin (Clevis pin) and a plastic bush.. which I think is not so good
It is possible to replace the phase 2 clevis pin and bush with a P1 roll pin.. but it requires some metalworking skill to do so ..
or look into the possibility of using an alternative.. like maybe the Doblo van item shown above.
The Punto mechanism solved the problem on my P1
car manufacturers try not to design one off minor parts for individual models .. so that led me to look across the FIAT/ALFA range to find something similar..