Ok...what noise is normal for a servo?

Or more accuratly, how much noise?

I converted my nexstar to dual aileron servos...when they are close their end point, they hum what seems like alot. And its strange, if i change the end point(increase), they dont make the noise at the same amount of rotation that they did before, but close to their new end...is this normal?

Theservos are the futaba 3151s (i think)