There is nothing the Skywalker and Organa-Solo families fear more that Artoo whistling innocently, because it means some one now has holo evidence of one of them doing something stupid, funny embarrassing or all three.
Worse than the innocent whistle is that menacingly jovial beep-chuckle he does.
Because that means it’s already too late, the information can no longer be contained.
(He made that sound when Mon Mothma related a story of a ten year old Leia freaking out and punching some poor clown at an Empire Day parade and by nightfall half the base knew. Of course, this meant Leia being jokingly proclaimed the patron saint of coulrophobics everywhere, but it also meant certain people in the Empire found out as well. Vader never says anything about it. He hates clowns too.)