I think you have hit upon the most likely scenario; the error occurs after it has been operating for awhile, and not during any specific operation. I think it is the case fan; if so, I think I can repair it myself (IF I can find someone who sells a case fan that will fit, that is if you think it is something that is non-terminal). Thank you for the info :):)
If you are handy, you can easily adapt a smaller fan to fit.
What I did when my two fans started to get really noisy was to pop down the street to my local electronics mom and pop shop and buy two cheap fans. They didn't fit because they were too small by about 1/2 inch in width.
I shaped two pieces of pine board ~3" long, 1/4" thick and 3/4" wide and used contact cement to secure them to the sides of the fan.
When dry, I popped the fans into place and...Whoooppeee! New fans.