Inside the trackball assembly are 4 rollers that touch the ball you see. When you move the ball, these rollers click and, short version, that's how the BB knows how far and in which direction you've moved the ball. It's the rollers you're hearing.
Posted from my CrackBerry at wapforums.crackberry.com
Probably the biggest thing you are hearing is the texture of the track ball contacting the rollers..... almost like a buzz/whirr sound when you scroll it fast. Sorry that's the best I can come up with to describe the sound effects.... lol.
CrackBerry is in no way Affiliated with BlackBerry. We take pride in our unbiased content, however do occasionally receive free products from vendors that we review or discuss. For more info click here.