Having a problem with the "alert" tone on startup

[Echidneys]

Hey, I'm new to the forum and it always seems like a specific question I have about my computer is hard to look up, so I finally decided to make an account for finding specific answers to specific questions.
On that note, I have a basic problem that probably has a basic solution, hopefully:
Whenever my MBP (late 2011) boots up, either after shutting down, or just restarting, I'll get to the desktop with no problem, finish loading (or almost finish), and then I'll get one single alert tone (Funk is default, but I have mine set to Sosumi), as if an invalid key is pressed. And then never again.

I don't have a problem with typing, though; no keys seem to be "stuck," the virtual keyboard shows no key-presses when I leave it alone, and no peripherals are even plugged in when it still happens. I don't touch any keys upon startup save for typing my password to log on, and after that, I can leave it untouched, and I'll get just one Sosumi beep. And only then. It doesn't happen after that.

Of course, it doesn't seem to be hindering anything, it's just annoying trying to figure out where or why the Mac is thinking I press a key right after my desktop loads up.
Anything that might trigger the Sosumi alert tone (like an invalid keystroke or something else) would be a good place to start poking around if anyone has an idea. I'd really appreciate it, thanks!

 

[satcomer]

Go to /Applications/Utilities/Console and look at the logs for startup to see what is causing the sound.

Plus what do you have in your System Preferences->Accounts, Your Account Name, Startup Items mini-tab and see if a program is causing the problem.

 

[Echidneys]

Sorry I took so long responding to this, I kinda forgot about it until today.
I followed your advice, and checked the login items, and found iTunes Helper (checked), Content Assistant Manager for my PS Vita (unchecked), and something called "CD Crossover something something" (unchecked), which had a little yellow triangle with an exclamation inside. I'm guessing that means it's a potentially harmful whatever it is, or it's "404/not found," which possibly triggers the Sosumi error tone?
I clicked the - to get rid of it, so next time I boot up the Mac, I'll see if that fixed it, which seems plausible after that. Thanks for your answer!