If the noise is just during DVD operation, than its normal. Remember that the drive in computer may rotate up to 52x faster than regular CD or DVD. It may reach speeds of 10000 rpm. Making quite a racket. Some CDs are a bit off balance which may make the noise worse. And of course please remember to avoid putting labels on computer CDs and DVDs. That extra unbalanced mass may end up destroying CD and even the drive itself.

After using computer for more than a year there is enough dust accumulation on the heatsinks and other surfaces inside to insulate them from proper cooling by airflow. As their temperature rise fans have to work harder (spin faster) making more noise. I recommend to disconnect all the cables take the computer somewhere where compressed air is available (compressed air cans will work too). Chose spot where the dust blown from the computer is not causing problem, take the cover of (usually left side looking from the front). You will find large heatsink on the processor (with fan) and potentially few smaller. Blow thru them from the side along the cooling fins. Also blow some air thru vents on the power supply (box where power cable plugs in). You can carefully blow thru the fans if dirty just hold them so they don't turn. After cleaning make sure that all fans move without drag or buzzing noise when gently turned. Best would be to take computer with cover off and connect it back, power on and verify all fans...

Beware, most of the rubber rollers of modern printers are made from urethane rubber. Contact with alcohol will actually ruin those rollers. Much safer solution is to use inexpensive reading glasses alcohol free lens cleaner.