Whenever I am drinking decaffeinated green tea our Grey will always beg until I let her have a drink. As she is drinking it she will stop, fluff her self up, grasp the rim of the cup loosely in her beak and shake her head back and forth so that a rattling noise is produced. After a few seconds she goes back to drinking tea and then back to her rattling display. She may repeat this four or five times. She only does this with tea, for example if same type of cup has juice in it she may drink some (usually acting as if she is disappointed that it is not tea) of it but she never rattles the cup.