I just got my first set of disc brakes when I got my new bike a few years ago. They don't make noise or squeal. Not even in wet conditions.
I really doubt the holes are the reason for your noise. They might contribute to it's unique sound, but they aren't the reason for it, IMO.
I've had plenty of rim brakes that squealed annoyingly too. Some even when dry. Many when slightly wet from damp roads.