Computer really noisy (fan I think?). Resetting PRAM and SMC does not work.

Question

Hi,
so basically, as the title says, my computer is making a really loud noise as soon as I turn it on, I am presuming it is the fan.
Over a month ago, my computer started to constantly crash, and about two weeks ago I took it to a Genius Bar for a diagnosis. The problem was a hard drive failure, so I had it replaced and yesterday I brought it back home.
However, after the fix, the computer now runs fine, but there is a really loud noise from the computer.
I have tried resetting the PRAM and SMC like some other threads have suggested, and none of them have worked. I have tried restarting, shutting down the computer and everything suggested.
The computer has never made this noise in this manner before.
Before the fix, the only time it would be noisy like this is when I wake it from sleep, just left it there (on screen requiring username and password) and it sleeps again, and if I wake it from sleep, then it would be loud like this, but that is solved by just simply sleeping the computer again and waking it, and it will be all quiet again.
However, now, it makes this loud noise as soon as I turn the computer on, and as long as the computer is not shut down or asleep, it will make this noise.
Before the fix, my OS was 10.4.11, but the Genius Bar had installed 10.5.8 on this hard drive.
Is this a software problem or a hardware one?
Anyone got any suggestions or ideas how to fix this?
I have called the Apple store I visited for this fix already, they push me over to AppleCare and AppleCare pushes me back to Apple store and they tell me they cannot provide technical support over the phone and would need me to take the computer back to the Genius Bar.
I am just not happy about this. I live almost an hour away from the closest store and I have university to attend to as well.
So I am hoping this problem can be resolved without having to go back to the Genius Bar, although all methods I have read so far do not help.
Thanks,
From Lelouch

Answer 1

Hi, well, the thing is, I am just worried that, while making other repairs, they might have damaged something unintentionally and probably do not know about it, which would not be documented.
My initial problem of a loud fan, after a hard drive replacement, was fixed, so I do not hear it anymore, but this wind-like noise is different. With the iMacs, I presume the fans are towards the top, which was where the loud fan noise came from, however, this wind-like noise seems to come from the bottom of the computer screen are, centre and right side (in the direction I face it), but not the left side.
I describe it as wind-like, but it sounds a bit like when the computer is on heavy demand and the computer loads, yet different as it is quieter and slightly different in pattern, and unlike the usual heavy demand noise which comes from the top, it comes from the bottom.
I actually installed a temperature widget after my last repair, just in case my fan was just simply tuned off and would not come on at all for the sake of fixing the loud noise it was making.
The temperatures read as follows:
HD: 31
CPU: 27
Ambient: 17
GPU: 36
GPU Diode: 46
GPU Heatsink: 36
HD Bay: 33
Optical Drive: 30
To my knowledge, these should be fine temperatures?
But the noise persists regardless. The noise can instantly be heard (providing that it is quiet enough) as soon as the computer starts up.
Thanks,
From Lelouch