When your budgie is making a soft crunching noise with its beak, it means that he is feeling happy and relaxed. Try for yourself and pay them a visit in the evening, talk to them and give them compliments. Don't speak loudly, but lower your voice so they don't feel intimidated. The budgies will return your care by twittering back at you and close their eyes every now and again. It won't be long before the crunching noise follows ;-) Which proves once more that your budgies need you around to be happy and to feel at home!