good suggestions so far! Definitely check on the keys on your keychain - that's an easy elimination as well.... Next try the putting pressure on the winscreen, and the fairings around it, and then the clutch lever as well... I hunted down sounds coming from engine vibes manifesting in all those locations.... Currently hunting one underneath the gas tank... lol, depending on how much you take your ninja apart, you kind of sabotage yourself a lot by reseating a lot of fairings and such....
Buddy of mine with a 2008 ZX-6R had this horrendous squeal at 10Krpm which sounded like one of those buzzers in cars which come on when you leave the door open, or don't put your seat belt on... Turns out it was his windshield being hit my a harmonic from the engine, and 2 turns in on one of his windhsield bolts and it went away...

It could be anything, so just start playing the elimination game.