Question : Problem: high  pitched humming noise in computer

I have a Compaq Presario 4402 that started making a high pitched humming noise and it does it all the time, even in sleep mode. I replaced the cooling fan but it still makes the same sound until you turn the power completely off. Please advise.

Thanks.

Answer : Problem: high  pitched humming noise in computer

Leave case open and see if any wires or cables are brushing the CPU fan (usual cause).
The only other things I know that could cause this are the fan in the power supply or the hard drive on the way out.
Be extremely careful but see if you can temporarily stop the CPU fan, Power supply fan & see if noise stops.
If so replace faulty unit.