@pawian, my immortal, unbeatable, super granny has only grown potatos, wheat and strawberries in the field, as far as I can remember, so I don't have any ideas anymore :)) ;)
You mentioned potato harvest. It might also produce that hollow sound in certain conditions. How?
The only thing that came to my mind was digging out potatos with the use of a hoe - the hoe hitting the ground makes a sort of blunt noise... Not sure about "hollow" though...