Hi, got a problem guys

wondering if u guys know whats up :p
cheers !
i recently noticed this purring noise coming from my PC im not sure what it is but its gets loud sometimes and other times its not there, it sounds like a Fan problem, i looked at my local hardware shop and say that it would cost me about £25 to replace both my fans, only thing is im not sure how they are connected and also if it is hard to do, its looks complicated and there are lots of wires going into the main fan which is also the main power supply :{ is this a big job, or are they just simple connectors much like a USB socket ?
cheers for help