See if it is catching on anything. Take the fan off the case and hold it in your hand and then power it up. I had one problem where the holes in the case were not sanded back down causing a bitch of a noise. Once i realised that it was the case and not the fan, a quick rub down, job done.
“Sorry but I cannot respond now, I am running a full virus scan on the internet.” -brandonwh64