Very Loud And Scary Noise From PC

I as far as I can remember have never had this loud noose before, but lately for a month or so? Ish? I've been having a series of loud noises from my pc, so loud that I can hear it even when playing BF3 with a headset! And so violent at times I can feel the keyboard vibrating lightly, and if I touch the PC it is vibrating decently softly.I may be wrong but it seems it started after BF3, and thought it might be the fan or my PC really struggling with the game, but it does it even when I'm not playing. It's sometimes just a softer sound, then will accelerate to a "burr, burr, burr" then go full auto into "brrrrrrrrrrrrrrrrrrr" occasionally very loud. So here's the REALLY scary part: while playing Skyrim a very weird thing happened: it stopped. The sound of the game looped into a "drrrrrrrrrr" and I had to hold down the off button. Nothing else did anything. I rebooted and tested it again...sure enough: 30 seconds in I had to do the same thing. I gave it one last chance and decided I would try to discern if it did anything else notable, but the third time it was fine. I left the game open a while to see if it'd do it again, but it didn't.So I'm scared. WTH is wrong with my PC? BTW it is kinda low on disc space...if that matters. Also the disc options are gone when I click on documents under the c drive and stuff...then again I had installed virtual drives..