And while you were in there, did you clean and get the dust out?
Sadly I'm finding desktop machines to be far from silent. And new noises do upset many owners but the sound you write about is not described in a way that would cause me concern about an impending failure.
I've just replaced my CPU's PSU with a new 500W PSU from a 300W PSU. The thing now is it makes this chainy, crickety chirping sound when the Processor is on load (e.g. scrolling a window, running a program) The noise seem to get louder as the load gets higher.
I've checked all the fans (Processor, Casing, PSU) and none of them was the culprit. The noise however was coming from somewhere in the Processor area. It never used to make this sound with the 300W. It just started after I upgraded my PSU.
P4 2.4 GHz
512 MB DDR
80 Gig Seagate HDD
Generic 500W PSU