It could be a noisy PSU fan, but keep in mind that disabling a device in device manager doesn't cut off it's power supply.
Open the case, unplug the power to DVD drive and switch the PC on.
When it starts making a noise **very carefully** investigate the CPU cooling fan any case fans and graphics cooling fan (if applicable). If they're not making any noise then you are down to two options. The PSU or hard drive.
Most PSU's are quite easy to open up and clean out - *be careful* if you do this. If it's the hard drive then a replacement drive should be the order of the day.