That would make sense, I got a cheap fan with this computer and it has caused very strange sounds in the past. I will try looking for loose cables and if nothing works there I will consider getting a new fan. I will keep you updated if it solved it. Thanks for your responseSounds like fan - and a bit of buzzing from HDD.
Potential issue could be one of the cables getting "into" one of the fans, this is quite common source of buzzing/noise. You could check for any loose cables inside your case - and if it's not the case, replacing the fan is another idea you may want to consider.