Does it have a fan? That would be my first guess. Power supplies can make some noise when one. Transformers will hum but can set up sympathetic tones. Was the noise always there? Fans will get noisier with age or dust accumulation.
I have an HP PSC 7210. As long as it is plugged in it seems to emit a hissing sound from the inside. The sound is faint (best heard in a quiet room, or if near the PSC) and it seems to originate from the inside of the unit.
Have other HP Printer/Scanner/Copier users experience a similar noise?
Any ideas on what causes this?