motion free falling body

Assuming the drip is regular then by definition the the start of a drop will be synchronised to the next drop e.g a Fixed time period.
Thus it will also be sychronised to the sound of the previous drop hitting the foil (what foil?) as the fall time will be constant.

It's a bit like asking why is the tic sychronised to the toc of an old clock!

Uless of course you didn't mean 'synchronised' but meant 'simultaneous' to the start of the next drop..?
If that were true it would be pure chance....but once the condidtions which cause the drip are set it is likely to continue.... and it would doubtless eventually go out of step.