For starters you don't go and buy $50 fans unless you KNOW that they are needed!
Having trouble determining exactly where the noise is coming from? Make yourself a makeshift 'stethoscope'. Get one of the 'fat' drinking straws, stick a small plastic funnel in the end of it, bung the funnel against your ear and use the straw to check various areas for noise. It'll help you pick out exactly where the clatter originates!
If the noise comes from the hard drive the drive is dying. Simple as that - they should make clattering noises!
Are you sure there isn't something contacting fan blades somewhere? A wire or cable that been pushed aside perhaps, and the fan blades rubbing against it?
It's not enought to 'think' the fans sound like they are working - you have to SEE that they're working.
I really do think that the money spent on having a repair shop inspect the system and advise you would be well spent. It is only a standard service charge for someone qualified to do that for you, and that s quite likely a move which could SAVE you money in the long run.
I have to admit that I'm a tad nervous about your situation. I'm usually quite good at detecting if someone is going to be comfortanle with doing their own repairs and you seem very hesitant and nervous about it all.
Here's a trick to try. Remove the hard …