My iPad Air 2 (2015) bugged out

So last night, my white front, gold color back ipad air 2 that i was checking the time on froze, and my phone wasnt available (it was dead) so i powered it off while it was charging. When it auto turned back on on its beautiful white background black apple logo it made a weird piano mac startup sound… please explain this to me!

Also its not an ipad air 3 dont look at the tags.