The backlight inverter in the Portable makes a high pitched whining sound when the backlight is on. Since converting mine to a solid state hard drive, that noise seems much louder (it's not being drowned out by the racket the old mechanical hard drive used to make).

Take Hen3ry's advice and get hold of a 6v lead acid battery - even if it's connected with crocodile clips. Just don't forget to put the lid back on the battery compartment after you connect it. There's a micro-switch that the battery compartment lid closes. No lid = battery disconnected and the 9v backup is in circuit instead.

Have you reset the power manager by pressing the interrupt and reset buttons at the same time?