How to close BalloonTip programmatically?

There are certainly better ways to do this. "Please wait" kind of feedback is best done with a progress bar or an hourglass mouse cursor. You can make it fancy on Win7+ with the Windows API Code Pack by displaying progress in the task bar button.

Anyhoo, you can pop a balloon by displaying another one with a short time-out or hiding the notification icon.

Try this:

Tray.Visible = true;

More info here.

Hope helps!