FreeBSD-specific buzzing noise.

[strings]

During the FreeBSD boot-up process, a buzzing sound presents itself. It comes from my computer case. This sound lasts until I reboot the system, but then it comes right back as the boot-up process begins afresh.

Now, this neither happens in Linux, nor in Windows for that matter. It's a FreeBSD thing. I wouldn't even need to install it for the sound to occur. I could just boot it from an USB stick and the buzzing noise would be there.

I've discovered a technique that allows me to fiddle with the sound, though. If I press Num/Scroll/Caps[Lock], the sound goes away for those few moments, as if it were produced by a deflating soccer ball and I could just put my finger on the valve and stop it from occurring.

Another thing that seems to interfere with it is the compilation process. As long as there's output text scrolling somewhere, there's silence. It doesn't matter if it happens in X, at the console or over the network.

I haven't made any modifications to the GENERIC kernel because I don't know what exactly is causing this. I've only added the NVidia and Intel HD Audio drivers/modules to loader.conf. It doesn't matter if they're loaded or not, the sound's still there.

dmesg output: http://dpaste.com/235208/

And these are the lines representing the point in time where the buzzing noise begins:

[...]

uhub7: <Intel EHCI root HUB, class 9/0, rev 2.00/1.00, addr 1> on usbus7
ad16: 610480MB <WDC WD6400AAKS-00A7B0 01.03B01> at ata8-master UDMA100 SATA 3Gb/s
hdac0: HDA Codec #2: Realtek ALC885 <= unloading and disabling it completely from the BIOS doesn't help.
pcm0: <HDA Realtek ALC885 PCM #0 Analog> at cad 2 nid 1 on hdac0
pcm1: <HDA Realtek ALC885 PCM #1 Analog> at cad 2 nid 1 on hdac0
pcm2: <HDA Realtek ALC885 PCM #2 Digital> at cad 2 nid 1 on hdac0
SMP: AP CPU #1 Launched!
Root mount waiting for: usbus7 usbus6 usbus5 usbus4 usbus3 usbus2 usbus1 usbus0
uhub0: 2 ports with 2 removable, self powered

[...]

What could it be?

 

[mav@]

As soon as you haven't mentioned speakers, I suppose it could be a coils in power converters of your motherboard. Power spikes could cause mechanical resonances in them. Compilation and other activity could change the consumption pattern, affecting these vibrations. If so, I am not sure that much can be done about it, except replacing motherboard. The only thing you may try is to change powerd running and/or C-states settings, trying to minimize the effect.

 

[strings]

No, there aren't any speakers involved. Yesterday, when I opened this thread, I forgot to mention that I tried to pinpoint the sound source by scanning each part of the motherboard with my ears. I also removed all other components from the computer case except the motherboard, CPU, HDD and PSU. This proved pointless.

In any case, since I've read your reply I realized that I didn't pay much attention to the PSU and today I did just that. I've opened the case, unscrewed the motherboard and the PSU, moved them as far as the cables permitted, powered up the PC and then thoroughly listened to each of them in all possible positions. Not far into this practice, I realized that as I was rotating the PSU, the sound would change its shape and/or volume. So, although it's not one of those cheap and poor quality models (it's a Fortron), the PSU is the culprit more than likely.

Turns out that the buzzing noise is still there whether I'm running Windows/Linux/FreeBSD. It's just that when the first two are running, the sound's pitch is quite high and I can't notice it from afar, thus wrongly concluding that it isn't there. On the other hand, when FreeBSD is running, the sound's pitch is much lower and it's really easy to notice.

After plugging everything back in and booting up FreeBSD, I was, of course, greeted by that familiar sound. Funnily enough, as I was typing this message, I noticed that nothing was interfering with the lovely sound produced by the fans. And, sure enough, after hitting NumLock a few times, I've concluded that it was no longer present. I have no idea how that happened. I just hope it stays this way.

Thanks.