yeah thats the sound of the hinges locking in place because if you dont lock the screen in place its really loose below that set angle, if those hingles fail your screen woould flop about lol.
The clicking really just means the connection is strong. Obviously with heavy use it will stop eventually but at least i know its gonna last longer than the none clickers.
My N3DS XL makes it and yes it has an IPS panel, im guessing the IPS models had a better build quality but the cost was more so thats why they are limited.
and yes my should buttons feel fine.
Also my DS Lite makes this clicking noise aswell when at the set angle and its just as load but hey i have had that DS since 2007 and its still looks new sept the spare styluse i chewed out