It is most likely the inverter card that runs the backlight to your screen. We have several thousand Latitude D620 systems at work, and a fair percentage do this. To check, you could try manually adjusting your screen brightness (FN+UP/DOWN on the D620) and see if the high pitched noise changes. If so, it's the inverter.

I recently got an I6400 (same as e1505 I think) that was pretty loud, making a high-pitched, electron buzzing noise. I couldn't pinpoint exactly what was causing it. Dell sent me a replacement, it's WAY quieter. I switched the hard drives but that didn't change anything so all I know is that it wasn't the hard drive. It's normal for any computer to make some noise but if you think it's louder then it should be contact Dell.