I figured they'd only be for level ups - but I keep getting them all the time. I get two or three per weapon per match, does it just alert you every few times you get a kill or something? Or every X amount of experience?

Every time I shoot down a UAV with my launcher I get the D-NN-NN-NNNRRT thing. And I only get, like, 50xp. I'm confused too.

the notifications are for lots of things. challenge completions as well as perk, rank, and weapon level ups that i've noticed. I don't know that i need that sound effect every time but whatever, i'm already being stimulated to the maximum extent by what's on the screen every second i'm playing MW3. what's a little more? and to answer your question, it looks like the weapons level by xp so in theory, kills in a game type like S&D would yield the highest potential xp for weapon leveling.

@MagikOvenMit said:

Every time I shoot down a UAV with my launcher I get the D-NN-NN-NNNRRT thing. And I only get, like, 50xp. I'm confused too.

Yeah, this is the biggest thing I'm noticing. I thought I had that thing at level 50 or something - then I check and it's level 3.