Alright, so my fan has become rediculously loud. And i've heard some pretty loud ones...anyways, what usually happens is, after the computer has been off for a while, i'll turn it on, and it will be running normal. but as soon as i do something processor intensive, the cpu heats up. and i expect the fan to go faster and a little louder, but not to the point where it never slows down again, even if i close everything and just let it sit there. i have to turn the computer off and wait a minute or two before rebooting. There are two basic sounds that i can explain...a white-noiseish noise that i have heard on other fans, and a turbine engine sound, that i hear when i travel on a plane. I've tried cleaning it out, but that didn't work....i also can't find any way to manualy contorl the fan speed...or even read the temperature of the cpu. any suggestions on what to do? any help is appriciated.

The solution may be to install another cas fan or two to carry that heat away ond out. The most common case fan size is 80mm. There are 25.4mm per inch if you need to know that. This is so you can figure out what your case will hold. the screw spacing on an 80mm fan is about 2 3/4" or about 70mm. When shopping for fans the CFM and the noise in DB are quoted. Variable speed fans are nice as are temperature controlled. If you aren't monitoring your system temps I suggest you do. Not sure what to recommend for your rig but I use Motherboardmonitor. This program may not be suitable for you. Mine has temp readings right in the tray next to the clock. The difference between the system and CPU shouldn't be more than 10 degrees C. and your system temps shouldn't be more than 10 degrees C. higher than the room lower is preferable.

It's staying hot most likely due to not enough ventilation in the case as eluded to by OtheHill. You don't really want to control the cpu fan speed manually as you may forget it's running slow to tune out the noise and it needs more air cause it's running hot. Proformance will degrade as a result of this. Cpu chips will stay hot for a bit so that accounts for the long time at high speed.
Bottom line is get more air moving thru the case. Cold air in, hot air out. Mad Dog fans I have found are high cfm and low noise. And quite reasonably priced.

I may be crazy, but I'm not stupid...

hmm...very interesting from both of you, thanks. but heres something else i noticed...right after i posted this, i took out the fan, and went to my room. i hooked up the fan to a dc supply of roughly 12 volts, and had it run. it would turn on, for about 5 seconds, and then turn off. so i decided i would put a heat source in front of it, basically a soldering iron minus the tip. but it did the same thing...so i dunno. but i hooked up a voltage meter to it, to check if its at 12v, like the fan says it takes. when the fan is not running, its a couple hundreths of a volt off of 12, and when it runs, i goes down to about 8.2v. which is to be expected of any motor really. so i wired some thin wire from the plug in the computer to the voltage meter, and turned my comp back on. i've done a few processer intensive things, and have been doing it for about 10 of 20 minutes, but the fan is still not even responding. i haven't heard a peep out of it. though i do know it is spinning, and the voltage meter reads 11.98ish. Im afraid i may be short circuiting something, but im pretty sure the computer would have told me when i booted up, since its kinda anal about having a fan working or not. heh. But about those temp monitoring programs, i was actually thinking of doing that, but wasn't sure if software alone would do it...i assume there is a thermometer of some kind in the cpu, even if its just an uncalibrated heat-variable resistor...but is it accesible from a software level? i'll look for that program you suggested in the mean time. thanks a bunch guys.

There is a sensor on the MBoard under the CPU.

Forgot this is Intel. I think the sensor is ingegrated into the processor. At any rate there is a sensor.

alright i tried your program, didn't have a dell dimension 8400 on its list...so it didn't work. i guess i'll keep looking around for stuff.

I have the same problem on my dell inspiron 8500 2Ghz P4. If i try to do something cpu intensive, the superfan (jet noise) turns on within a minute or two and it never stops, even if all apps are closed. One thing i noticed when the fan kicks in, the cpu speed drops from 2Ghz to 300Mhz and can never go higher. This leads me to believe it is a speedstep problem, or something about the cpu conflicting with windows. In any case, it requires a hard reboot to get the cpu back to 1.2ghz-2ghz and turn off the superfan.

OK Guys, I just ordered a new fan from Dell, and that didnt solve the problem that you all are describing. I get that same Turbine sound, scares me sh*tless... but nonetheless, it still doesnt work correctly. So, what I am doing is ordering some Arctic Silver 5 and am going to replace the Heat Sink Grease that came on there, as it was all Dry... and I will tell you all if I get that to work