Maybe something is wrong with the power supply? Usually I hear humming/buzzing noises when I'm close to a power line. (I wait for my bro at 12am in a lonely driveway and there's a power line there that yells at me... that's how I know.)
Try being more descriptive. Try to guess what it is, because it's really hard to say something from just a sound.
I've had my Slim for over a year and I've noticed the same thing since I got it. I play it all the time and nothing seems wrong.
Yes, I did create that game on the Vita you probably never heard of.
My 500 GB slimster does the same when it's powering up.makes like a humming or buzzinf sound,In my case i think its a fan or somthing,thats what it reminds me of anyways,After a few moments it stops and works fine,Hope ist not about to die..lol..just got about a yr ago...