What is my computer doing?

[drew]

I have an older, home-built computer (assume >5 years old, for the sake of discussion) in my "home office."     The onboard sound card is hooked up to one of those amplified Cambridge Soundworks  speaker systems with a subwoofer and two little satellites.  I turn the computer off when I'm not using it, but I never bother to shut off the sound system. 

Recently, with the computer off, the speakers will randomly start emitting a steady, low-frequency THUD-THUD-THUD-THUD, about 4 thuds per second.  If I unplug the computer from the wall, it stops after a couple seconds.  What's going on here?  Is the computer's power supply crapping out on me?  If I remember correctly, it has an Antec power supply, one that was pretty good quality at the time I put the system together.

[EL34]

I can't say for sure, but the mother board in my computer still has power after shutting down.
There is a green led that is always lit

pull the plug on the back of the pc and see if it stops.

that won't fix anything, just curious

[Shrapnel]

ALL ATX computer systems will have power running to the motherboard, even when seemingly off. If your power switch is connected to the motherboard instead of directly to the power supply, it is ATX, and ATX has been sold for about a decade or so now. The old AT type stuff would be a [80]286 CPU (Well, connectivity wise, even back to the original IBM PC and it's 8088) to a [80]486 and to a degree, the Pentium processors. (That's the time period that the switch from AT to ATX started taking place.)

It's possible even for the best power supplies to act flaky if they go bad. A good brand just means it shouldn't go bad for a LONG time (it does and can happen, not as often as the el-cheapo power supplies though. AND those el-cheapos can have a bear of a time even coming close to rated power output.)

It's more likely a problem in the sound card though as the rest of the computer seems to operate fine. If you stick in another one (sound card), or enable and hook up and built in audio card functions, and it goes away, I'd blame it on the sound card. I've got an Audigy card here (not on my new build here though) that it doesn't matter what drivers (original, or the newer ones that support OpenAL) the volume will have a tendency to change on me every so often.

[simonallaway]

Forgive me as this might seem on a tangent, but do you own a GSM cellular phone?

Their radios interfere with most audio systems and have a very distinctive pattern to what I can only describe with words like 'buzzing, pulsing, fuzzy morse code' (i know, so scientific). In the old days of CRT computer monitors you'd even see the picture start to shake violently as a call was coming in.

A nice solution is to place the phone on the kind of anti-static bag you get hard drives in. I think it's acting like a mini Faraday cage. Any GSM phone i've tried it with shuts up almost instantly.

[Shrapnel]

Forgive me as this might seem on a tangent, but do you own a GSM cellular phone?

Their radios interfere with most audio systems and have a very distinctive pattern to what I can only describe with words like 'buzzing, pulsing, fuzzy morse code' (i know, so scientific). In the old days of CRT computer monitors you'd even see the picture start to shake violently as a call was coming in.

A nice solution is to place the phone on the kind of anti-static bag you get hard drives in. I think it's acting like a mini Faraday cage. Any GSM phone i've tried it with shuts up almost instantly.

Now there is that possibility too, AND this suggestion is cheap to try, cheaper than a new sound card, or the time to remove one, and enable on-board sound.

Drew, I'd try this first then, IF it fixes it, then you know your cell is at fault... then again... if someone else's cell was in close enough proximity... (I'm not sure exactly how close it would have to be.)