I'm on a desktop PC with Windows XP. I assume it's the hard drive because it's a mechanical sound. It's a hard sound to describe but imagine driving at high speed with hard wheels on a brick or cobblestone street. Or on a railroad track with two wheels running across the ties. It's a very aggressive sound. It doesn't always happen which made it take me a while to pin it down until I am pretty sure it's related to Firefox. I thought it was Google or something else but after experimenting, the noise stops immediately when I close Firefox. CPU usage on the task manager is variable, but I've noticed the Firefox process spikes in rapid waves between 2% and maybe 25% or more while the noise is going on. Otherwise, my CPU usage seems to stay under 10%. Would it help to uninstall and reinstall Firefox?