avayaitup,
So, just to clarify... when you say "noisy", do you mean it sounds like it's running at full speed instead of low speed, or it sounds like something is broken?
In context, I believe it would be best to use the term, "noisy" if it sounds like it's broken - possibly a slight grinding sound, that sort of thing. The use of the word, "loud" would be better to refer to a fan running at full speed rather than slow/low speed.
Well, that's my opinion, anyway. I think it would help clarify things.
If the fan is truly "noisy" in other words making non-normal sounds, then it'd be best to just get a new fan, and replace it. Go to one of the many sites which sell computer parts, and order one that fits your CPU, and that it fits your heatsink as well.
If the fan is just running at full throttle - not really making any additional noise, but just sounding louder than mormal, then it may very well be an issue with some other hardware (or software).
And when you say it's going to sleep.... do you mean it goes into hibernation, or just sleep? How sure are you as to what it's doing? What symptoms/details make you think it's one or the other.
One way to test, maybe fix the issue, if it's related to hibernation is to initially disable hibernation. See how it does with hibernation totally disabled. Then if it seems to fix it, and you want hibernation, try re-enabling it, and see what happens. It'll take some time, obviously, but it should give a real world picture as to whether sleep/hibernation has any role in the events.
Another test, if it's not hibernation, would be to change the power settings. Change how long it takes for the hard drive to go to sleep for instance... or disable "sleep" altogether by running it at "high performance" mode, or whatever it was called in XP - I don't recall at the moment.