In Podrima and Prizrenski Podgor they consider this transformation occurs if these ground fruit have been kept for more than ten days: then the gathered pumpkins stir all by themselves and make a sound like ‘brrrl, brrrl, brrrl!’ and begin to shake themselves. It is also believed that sometimes a trace of blood can be seen on the pumpkin, and the Gs. then say it has become a vampire.

Source: Vampire pumpkins and watermelons – Wikipedia