So, this question has been bugging me a lot, I used to have a few lovely little toys (well, not so much toys as a pentium pro and a really old Intel 80386DX microprocessor)
What I noticed is that when the CPU's are under load they generate a most delicious noise which reminded me of when I used those computers for actual things (the 80386 had DOS 6.2 at one time and I ran a small unix server on it a few years later, the pentium ran windows 95 and then later windows 98+ and even later I somehow barely got xp running on it), I almost dare say it's nostalgia.
Anyway, that noise made me curious, what causes it?
My first hypothesis for this rumbling/humming sound is the fan, so I tried removing every last fan, the noise didn't stop, it was actually easier to hear when the whirring of the fans wasn't there, I then spent hours testing each piece of hardware and came to the conclusion that the CPU itself was making the noise and then decided that I was happy with my conclusion for then, I recently remembered it and I got curious as to what generates it.
I don't own those machines anymore (alas I had to get rid of them due to moving) but I have observed that newer CPU's make the same noise, just a lot fainter and more subtle, so I do think it has something to do with the underlying architecture.
Anyway, before my rambling carries off too far… What causes the noise and is there any correct terminology for it?
These questions are derived from a few hours on google and getting nothing but people asking silly questions such as "Why is my hdd ticking?" or "Why is my CPU producing a high pitch noise?" (the latter is closer but not what I am looking for as the noise is low pitch and quite distinct, if you've heard it you should know what I'm talking about)
I want to know this for a couple of reasons, curiosity is one but I also want to utilize this sound effect in a project of mine.
Now, some of you may point out that CPU's have no movable parts and are thus incapable of generating noise, then what could possible causes be?
I tried removing everything redundant from those machines and the sound prevailed, the only things I didn't remove or replace was the mobo's and the cpu's themselves (because finding such old hardware proved harder than expected. oh and this part is what really sparked my curiosity as to what was generating the sound as the fans were silent when just powered on, and the machine kept making noise without them (I ran them at short 1 minute intervals))
Best Answer
I have experienced this on all laptops with Intel processors about 1-2 years of age. It is indeed coming from the CPU itself. I have read that it has to do with overclocking (I think it's called Intel Hyperthreading Technology), which is can be disabled in the BIOS. If enabled, the processor can overclock itself at high loads.
Also, it could be the C-states of the processor. These are power-saving states and when the voltage drops to zero or comes back, the individual circuits and connection pins inside the cpu heat up. The CPU does contain a lot of microscopic transistors that will create noise when the voltage is altered. You know this from power adapters to phones, etc.
Paging through the 200+ pages of hardcore technical specifications of the latest Intel i7 chip, I discovered the following. I cannot claim to know whether there be any connection to the issue at hand: "The system memory controller incorporates a Data Scrambling feature to minimize the impact of excessive di/dt on the platform system memory VRs due to successive 1s and 0s on the data bus. Past experience has demonstrated that traffic on the data bus is not random and can have energy concentrated at specific spectral harmonics creating high di/dt which is generally limited by data patterns that excite resonance between the package inductance and on die capacitances. As a result the system memory controller uses a data scrambling feature to create pseudo-random patterns on the system memory data bus to reduce the impact of any excessive di/dt."
di/dt has to do with instantaneous voltage (i.e. C-levels). Remember that every circuit is a "speaker" because the induced magnetic field around the wires/connection pins make things vibrate. Vibration=sound.
This has to do with simple harmonics, and that might be why my processor of 3.8 GHz can only be heard under certain loads, because of overclocking and thus the alteration of these harmonics. 3.8 GHz = 3.8x10^9 changes of current per second. This must mean that at some microscopic level, the CPU vibrates as well. I guess that under normal or idle loads the CPU throttles down to a lower frequency than 3.8. At higher loads, the frequency of the electrons through the CPU accelerates to 3.8 GHz, which makes certain parts inside the CPU "vibrate" at some macroscopic level, and therefore we can hear it as a sound.(those of us who have those frequencies of our hearing to spare — human age is probably also a factor here :). Those vibrating parts might be the transistors or whatever.
Also worth reading: https://electronics.stackexchange.com/questions/126502/how-can-purely-electrical-circuits-emit-sound