A google search reveals a lot of talk to problematic noises. However, what’s that sound that my tower makes when its running normal?
That light, sort of “crackling” noise coming from inside. You probably know what I’m talking about. Every desktop does it. What is my computer doing that makes sound? Is some sort of sparks flickering over the processor?
It’s the disc drive reading/writing.
And the noise is from the mechanical motion of the drive’s read/write head as it rapidly moves back and forth over the hard drive’s platters. It’s not sparking or anything like that.
It’s the hard drive. Spindle drives have moving heads thtat scan the surface of the disc platters in order to read data. That’s what that noise is.
Curse you lazybratsche. Why can’t you live up to your name?
Oh, I do. Why do you think I spend so much time here? 
Could be fans too. The noisiest part of my computer is the fan for the power supply unit.
Here’s a video of what a hard drive is doing to make that noise. Note that when it’s enclosed in the case and box it’s a quieter and more hollow rattle and clunk.
Thats more of a constant hum or buzz. The OP was asking about the “crackling” noise that the hard drive makes intermittently. Some computers also have a LED that flickers to the beat of the hard drive activity. You will also notice the sound becomes more constant while copying large files or installing applications.
In my experience of using Windows, disk thrashing seems to be a constant feature, even with as much RAM as the OS can address. 
That may be due to Windows “optimizing” its swap file, which is something that Windows often does when it doesn’t have anything else to do. I wouldn’t really call it “thrashing” since it doesn’t usually hit the disk that hard when it does it.
Nice vid. It also illustrates why file fragmentation hurts performance so badly, the more fragmented the files the more mental that read head is going.