:-D I had this EXACT problem happen to me about 4 months ago. Thought I was so clever getting the dust out of the PC and the fan.
Then it started making this horrific noise everytime it rebooted. Which most of the time would last for hours.
What has happened is that your fan is unseated just a little bit, but enough to be really annoying. All you have to do, is open up your case again, and make sure that the fan is seated solidly, that way it won't vibrate.
The other way, which by the way I do not recommend, is to smack your computer every time it start buzzing ?

. Usually makes it stop, but could cause other things to come loose.
I'd definitely go with the first option.
Let us know if that solves the noise,
TTFN
T