Hi, It could be the hard drive, Other options are:
If the noise is only there on a cold start and goes away shortly, it could be a fan with a bad (Dry) bearing.
If there is a disk left in the CD/DVD drive it could be coming from there.
The way to pin it down is when the computer is cold try opening it and then turn it on. See if You can pin the noise source down. One way to tell if it is a fan going bad is before You turn on the computer use Your fingers to prevent it from spinning, Then power on and see if the noise is gone. If not repeat. I believe You computer has two fans as well as a fan in the power supply.
For the Power supply fan use something non conductive like a wood pencil stuck just far enough into the power supply grill to keep the fan blades from turning.
Always stop the fans before applying power to prevent damage to the fan or Your fingers.
Main causes of noise when turning on a computer that go away after a few minutes is a bad fan bearing.
There are a limited amount of parts in a computer that have moving parts and can cause noise. These are Hard Drive, CD/DVD drive and Cooling fans.