To make your selector click, buy a mp5 or m4 selector switch, and use the spring and nub and install it in your umg selector.
As for your mechbox noises, open it up and check for damage to the cylinder head or piston head. Also if you don't have a bearing spring guide (and have a regular non bearibg spring guide) your gun will sound like ass. That could also be it.
Every word ever written will fall short, of it's intent
Even sung, or spoke, or screamed, they will betray what they had meant.