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 are turning without excessive noise (replace fans as necessary). Next, turn off power and replace cover.