I did the same as well a few months ago and now it roars at times. Maybe you could leave the side open and have a look at your graphics card fan when your PC is on. It's likely that the fan isn't working as well all the time and if it slows down at times like mine, it produces a loud roar. It does worry me though since one day, it might stop altogether and my graphics card could fry. Looks like I will need a new fan and heatsink soon and you might need one as well. There are some nice and quiet fans though and I did see a noise-free one before somewhere.