Wierd PC noise, resonsance freuenceny?

[iratemonkey]

Weird PC noise, resonance frequency?

I have this wierd low level hum hum hum noise that comes from the computer, I'm thinking one of the fans, either cpu fan, sys fan, or gpu fan but I don't know which one. When I'm setting down, randomly I will hear what sounds like a low level humming noise that increases in intensity and frequency (like the noise those AC units makes in the summer) almost like there is some tessellating resonance frequency going on where the vibrations and fan "rev" noises are rhythmatic.

Then every time I put open the computer case while the computer is running to investigate and to get a closer hearing of it, the rhythmatic noise seems to magically go away.

My theory is that by opening the case, either by changing the structure of the case or interrupting the airflow patterns that had developed that I got rid of the by disturbing the resonance frequency

Once I put the case back, and I sit down, after a while the noise comes back.

Is my ears imaging things and I need to go to the doctor, or is it my computer?

 

[hjj174]

If it was resonant frequency, the part would probably destroy itself. Resonant frequency is when the vibration's frequency, caused by something typically mechanical, matches the frequency at which the molecules of the substance are vibrating. There would be a much louder noise caused by this when what you are describing.

 

[dish_moose]

Resonating does not mean it will destroy itself. If that were the case there would be a lot of dead people from singing in the shower. Is there any amount of dust on your fans? I had one fan do something similar - took me a while before I found the culprit - turned out to be one of my GPU fans.
-Bruce

 

[iratemonkey]

No my parts are all new, strange thing is I want to catch it in the act, but everytime I open the case the noise seems to go away!

 

[hjj174]

Have you ever seen a wine glass shatter from an opera singer? That glass breaks because it is at resonance. Most of the time, resonant frequency of objects are at such a high frequency that they are hard to reach. It could be a bearing in a fan or just something slightly loose. I know the feeling of being unable to locate noises. I'm kind of experiencing it myself right now.

 

[dish_moose]

Resonance can also occur at low frequencies as well - http://www.youtube.com/watch?v=j-zczJXSxnw. Tuning forks also resonate - they do not destroy themselves. You are talking about destructive resonance.
Vibration could be caused by fan defects causing a "wobble" like an improperly balanced wheel on your car. Can you try disconnecting one fan at a time to see if you can locate the problem?
-Bruce

 

[iratemonkey]

Turns out it was the harddrive. I had it upside down.

 

[dish_moose]

Hehe - too funny, most HDDs don't care which way is up. Maybe when you turned it around it was more securely fastened.
-Bruce